RadialContext - Replaces the standard context menu with a radial one. (gestures)
BugMeNot - auto fillin for web registration
Allow Right-Click - defeats some sites right-click blocking
CookieCuller - extended cookie management - flush all cookies except selected ones
SwitchProxy Tool - switch between multiple proxies
Diggler - advanced url manipulation, including go up a level
Adblock Plus - self expanatory
Adblock Filterset.G Updater - self expanatory
Book Burro - check prices on multiple sites
Greasemonkey - if you don't know about Greasemonkey....
Urban Dead Homing Beacon - for urbandead.com, text zombie MMORPG
udtool - for urbandead.com, text zombie MMORPG
Linkification - make text links into clickable
CustomizeGoogle - lots of little things