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

• Essential Firefox 3.6 Add-ons
  Note: Some of these are still in the experimental phase and require manual updating! These are marked with an asterisk (*).
  • IE Tab 2
  • 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, SearchPreview, Google Shortcuts, Integrated GMail
  • Multi-Firefox synchronization: FEBE, Xmarks, OPIE
  • Password management: PasswordMaker, Secure Login
  • Patch For Library*
  • PDF Download
  • Tab Mix Plus
  • 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
  • Panda Cloud Antivirus
    The latest, greatest free antivirus software. Best installed on computers that are already clean.
• 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!
  • FileZilla
    The other great free FTP program. Server version also offered for free.
  • 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 as well as transparency in maximized windows in the default Aero theme.

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