Firefox

• http://www.noscript.net/whats - Part of the "It's my fscking computer; get your grubby JavaScript off it" thread.
• http://www.bugmenot.com/ - Bugmenot -- bypass compulsory web registrations.
• http://roachfiend.com/archives/2005/02/07/bugmenot/ - More Bugmenot
• http://chrispederick.com/work/useragentswitcher/ - User Agent Switcher -- handy for doing web debugging.

Also see the "Firefox goodies" thread at http://mail.gnhlug.org/mailman/private/gnhlug-discuss/2005-December/thread.html