Must-Have Free Software

General Utility

• 7zip: In case anybody doesn't know, the freeware compression/decompression utlity of choice for zip, rar, etc
• Abyss Web Server: Host from home.
• AutoHotkey: Very powerful open source tool can script+customize macros, hotkeys, everything input related within the Windows environment right down to mouse clicks.
• Bulk File Changer: Batch utility for altering file attributes.
• Cabos: file sharing client for the Gnutella network. Not as much customization as the shareware Limewire but it does the same job.
• CDisplay: CBR/CBZ format sequential reader, which are rebranded zip/rar extensions popular for image archives such as comic books, guitar tablature, etc.
• Cheat Engine: Extensive memory editor/manager. sort of like Game Genie/Codebreaker for PC.
• ClassicShell: Restores many of the Windows features that were stripped for Win7 including disk free space on Explorer's status bar, restores toolbars, the classic file copy dialog, etc.
• Crosshair: Replaces the mouse cursor with an origin point along x and y axis and is surprisingly more handy than you might realize.
• CPU-Z: Simple utility for monitoring system components+performance
• DC++: File sharing client for the Direct Connect network that was providing a superior experience over the official Neo Modus client for years.
• DriverSweeper: I've been using this for some time to clear old drivers from systems.
• EaseUS: Several great products like Partition Manager and Disk Copy, generously free for personal use.
• EasyBCD: You would want this in order to manage boot loaders and restoring+repairing broken entries under Windows.
• FileTypesMan: All the features Microsoft stripped from file extension management in Vista and 7, and all the features they should have implemented in the first place.
• Filezilla: Fully featured freeware FTP client.
• Flare: Decompile Flash swf.
• Go PlayAlong: Shareware guitar tablature player for Guitar Pro formats. Just recently got into it, but I love the mp3 sync feature for backing tracks.
• HJSplit: Who could live without a handy file splitter-joiner?
• HTTrack: Downloads web site resources to generate a mirror for offline browsing. Use responsibly.
• inSSIDer: Seems to be the wifi scanning tool of choice now days.
• JoyToKey: J2K can map controller input to keyboard keypresses, useful if a game fails to provide joypad support when they should have. Results may vary.
• Less Msiérables: Extracting contents from a .msi file.
• Microsoft Power Toys: More from the "why isn't that built into XP by default" category, includes utilities like CmdHere that will add a directory context to Explorer for opening a command prompt at that target location.
• Opera: Yeah, all those features people rave over Firefox? Opera was doing them years prior.
• PeerBlock: Monitors connections and blocks many of them based upon lists of registered IP ranges belonging to various government, anti-p2p, etc. agencies.
• PowerMenu: Adds 'always on top' and other functions to Windows. I think I got this for some emulators lacking it.
• Putty: Excellent Telnet+SSH client with great functionality. I've been using it for my *nix shell as far back as I can remember.
• ShellExView: If you want to cut down on some of the unnecessary shell extensions crowding the right click menu under Windows Explorer, this is a handy way to do it without manually editing the registry.
• Soulseek: Used it for gathering some hard-to-find music way back. Because it runs off users shares, it doesn't have the shorter lifespan of most torrents but it's more manageable than Limewire and those types.
• Sysinternals: Every one of these should come packaged in Windows by default. All the utilities you may want as a Windows power user are here including Process Monitor and Autoruns.
• Utorrent: My favorite bittorrent client to date. They're not kidding about the lightweight and efficient part. Also customizable and fully featured.
• WatchCat: Really old program for a few functions like toggling visibility of windows to hide them from observation. It was great for hiding the omnipresent banner ads in some software through the 90s. I'm sure there are some potential uses for it even today. Still works under Windows 7!
• WinDirStat: Very useful utility builds a graphical table of any drive or directory tree in order to quickly identify consumption.
• Windows Resource Hacker: Can import+export the resources of Windows formats including exe, dll, ocx, etc
• XN Resource Editor: If Resource Hacker isn't up to the job any more, there is Resource Editor.
• XVI32: My preferred hex editor.
• 7zip Theme Manager. I like this one in particular
• DOSBox: fully emulates dos, including sound drivers, mouse, etc.
• Foxit pdf reader: I use this when the adobe one opens too slow. I don't think it supports as much specialised functionality, but when your files are plain pdfs and you don't like waiting then you're good to go. Metamorphose: similar to bulk file changer
• Imgburn: disc recording software, though these aren't as useful as they used to be because of memory sticks being smaller, higher-capacity, and rewritable. Sometimes you need it.
• mediainfo: context menu option to review the media. Shows video track(s), audio track(s), and data like codecs, frame rate, data rate, aspect ratio, audio sample rate, etc. An enthusiast's wet dream, really.
• Superfinder XT: get back the ability to actually do a real filesearch in windows 7 (vista, too?).
• Teracopy: for when you need to copy a lot of files; like 2 GB to 2 TB at a time. Can replace default copy handler, but I usually disable it and use it only for large actions. Performs CRC32 after copy if you tell it to (can make it default in options). Tells you the transfer rate as well.

Email

• Thunderbird: not exactly a secret, but it's free and it's awesome. Set accounts to imap and you're good to go. I have 5 accounts on my thunderbird and it makes checking mail painless.

Graphics

• Microsoft GIF Animator: Classic, no frills tool for creating gifs.
• Paint.NET: an excellent replacement for mspaint. If you can't make a solid and bulletproof argument on why you need photoshop, you can use it in place of photoshop.
• Inkscape: vector graphics editor

Productivity

• Calibre: convert between text types and ebook formats. Notably useful is converting epub or html to mobi.
• PDF Creator: Create watermark-free PDF files. Do they even sell those stupid programs anymore? Plus some programs (like chrome) have built-in save-to-pdf functions. This creates a printer that you can print to, and then save to PDF for whatever else doesn't support it.
• LibreOffice: maybe a bit quirky when compared to office 2003 (which is obviously-heavily-"inspired"-by), but it's free and it does the job.
• Notepad++: just a nice text editor. Best replacement for ol' notepad.exe I have found yet (doesn't mean it's the best, though). Does tabs, highlights brackets, highlights commands, does nesting, etc. It also supports plugins if you want even more than what is built-in.
• GraphCalc: a decent calculator, and tabbed 2D/3D graphing.
• Foxit: Freeware PDF reader as the less annoying alternative to Adobe.
• Programmer's Notepad: My preference for a text editor+Notepad substitute. All the best features like code differentiation formatting, tabbed organization, managing projects.
• Sumatra PDF: Seems to be a popular lightweight reader. Has one .dat file for preferences, doesn't require an installer and supports numerous file formats.

Video & Audio

• Audacity: multi-track audio editor. Supports a ton of filetypes. Very excellent.
• AVI cc changer: In case you need to alter the identifier in video files
• AVI Mux: Utility for managing multiple audio streams in video files. For example, adding a commentary track from an mp3 file to an avi video.
• virtualdub: a way-more-specific-crowd kind of tool, but I'm going to list it. You can get plugins so it supports almost all file types, but it usually works in AVIs. It's great for a few things (stripping sound, uncompressing video, chopping streams, Video->image sequence, and some other stuff).
• CCCP: The Combined Community Codec Pack, which I've found immensely useful for installing on other people's systems so I'm not plagued by requests to troubleshoot+resolve every little codec issue they encounter.
• DVDx: Rips video files from dvd source.
• Exact Audio Copy: Once upon a time CD ripping and encoding wasn't available in every other program. Still seems to be used by many people.
• Gspot: Tool for gathering information on video files and infinitely useful for troubleshooting codec issues
• Handbrake: easy x264 encoding
• Media Player Classic Home Cinema: My own preference for video player, and immensely superior to WMP.
• MP3tag: Probably the best and fully featured tag editor for media files around, far better than Winamp and iTunes.
• Real Alternative: Play .rm files without having to install the worst player ever.
• VLC: media player. Just get it.
• VCD Gear: Various functions for mpeg & vcd formats
• VideoLAN media player: A decent player to install on other people's systems because I still hate getting phone calls over mundane issues like codecs.
• VirtualDub: Great video processing utility for encoding and editing videos.
• Winamp: Still my favorite audio player since the 90s, and has grown to continue supporting everything I need with plugins like ml_ipod and the new Bento interface.

Emulation

• VirtualBox: create virtual machines and run alternate/old/sandboxed operating systems. Good if you don't trust some program. Also good if you want to use linux programs like LaTeX in windows (I found it was much easier to install LaTeX in a virtual machine than it was to get it working in windows a while back). You might need a more-modern cpu (one that supports virtualization) in order for this to work. It also isn't great at win95/win98 (pegs out a CPU at 100% and is laggy as hell).
• Daemon Tools: Even after going commercial they're still the optical drive emulation software of choice as far as I know.
• DeSmuME: Seems to be the only major Nintendo DS emulator in development as of 2012. Not sure why anyone would want it when a NDS+flash card are so cheap though.
• Dolphin: The only solution for Gamecube and Wii, because you haven't enjoyed New SMB until you've played it with a Sony controller.
• DOSBox: DOS emulator for Windows, because Microsoft can't be arsed to provide real backwards compatibility for their own OS legacy.
• ePSXe: Definitive PSX emulator.
• KEGA Fusion: There are quite a few SEGA emulators out there. This one is best.
• Hoxs64: Very faithful Commodore 64 emulator.
• MameUI: Formally Mame32, Windows port of Mame.
• NNNesterJ: Granted NES emulators are as abundant today as Tetris clones. But this one seems to be a little bit better than the others.
• PCSX2: The foremost Playstation 2 emulator has developed enough to become playable for most games. A modern gaming system is absolutely required. Systems with integrated graphics adapters need not apply.
• Project64: N64 emulator
• Red Dragon: If you wanted to emulate VirtualBoy (for some reason)
• ScummVM: PC adventure emulator for just about every known platform.
• SSF: Fully functional emulator for SEGA Saturn. It does include an english language option within the program.
• Stella: Atari 2600
• Visualboy Advance: Emulates the entire line of Gameboy systems.
• x360ce: Emulates the Xbox 360 controller Xinput.
• ZSNES: SNES emulator for Windows.