FYI: This page will always be a work-in-progress.

• Essential Firefox 3 Add-ons
  Note: Some of these are still in the experimental phase and require manual updating! These are marked with an asterisk (*).
  • All-in-One Sidebar
  • Bookmark tools: OpenBook, Patch For Library*
  • Coral IE Tab
  • Developer tools: ColorZilla, FireFTP, Web Developer
  • Download tools: Download Statusbar, DownThemAll!
  • Feed reader: Brief
    I’ve recently decided to switch to Google Reader, but if you prefer to use a Firefox extension, I personally prefer this one over Sage (but that’s just me).
  • Google tools: Better Gmail 2, Gmail Manager, GooglePreview, Integrated GMail
  • Multi-Firefox synchronization: FEBE, Xmarks, OPIE
  • Password management: Secure Login
  • PDF Download
  • Tab Mix Plus
  • Tiny Menu
  • translator
  • Ubiquity
  • URL Link
  • Yet Another Smooth Scrolling
• Portable Apps
• Developer Tools
  • IETester
    An IE shell which includes the resources necessary to emulate IEs 5.5, 6, 7, and 8, as well as whatever version is installed on your system.
  • Notepad++
    The ultimate Notepad replacement. Includes syntax highlighting for just about any programming language you can think of.
  • WinMerge
    Compares the differences between two files — even generates patch files.
  • XAMPP
    An all-in-one package of various servers used in web development.
• Games
  • Digital Paint: Paintball 2
    An FPS-style paintball game based on the Quake 2 engine.
• Miscellany
  • Mozilla Thunderbird
    An email client made by the same organization that brought you Firefox.
  • URL-Run
    Adds a protocol handler to the system that can call programs & batch files with parameters.
  • Taskbar Shuffle
    Gives XP or Vista the ability to rearrange taskbar buttons with a simple drag and drop.
  • Windows Vista Cursors
    Extract to C:\Windows\Cursors
  • Windows Vista Sounds
    Extract to C:\Windows\Media
• Security
  • A-squared (Anti-Malware) and Online Armor (Firewall) Bundle
    The best of both categories in one download! Both are must-haves for keeping your PC protected from all the nasties out there.
    (Stand-alone installers are available for A-squared and Online Armor.)
  • Avira AntiVir
    The other best anti-virus program. Super accurate, like A-squared, but the downside is the ads it feels like it has to show you.
  • Avast!
    The second-best anti-virus program. Doesn’t show you daily ads like the one above, but it does require you to register your copy of the software every year. Not to worry, though — this registration is completely free. I suppose it’s just so they can keep track of how many people use their software or something.
• Utilities
  • 7-Zip
    An easy-to-use file compression program that can read and write many kinds of compressed files.
  • BitKinex
    Talk about feature-rich! This file transfer program that supports FTP, FTPS, SFTP, FXP, HTTP & HTTPS (including WebDAV), and SSH – and it’s free! Wow!
  • Ext2 IFS
    Enables you to read & write to Linux drives using the Ext2 or Ext3 filesystem from within Windows!
  • VistaGlazz
    Enables 3rd-party themes in Windows Vista, and also enables transparency in the default Aero theme in maximized windows.

There’s another good list of freeware here.
…and here are some web design resources.