Adblock Plus: blocks ads
bug48929: prevents the "tearing" action that opens a new window when dragging a tab downward
DownloadHelper: lets you download pretty much any video off any site
DownThemAll!: download manager, works great for porn interesting pictures
Extended Copy Menu: gives a context option "Copy as Plain Text," shortcut control+alt+c
Extended Status Bar: more status bar info
Firebug: useful web development tool
ImgLikeOpera: disables images by default, allowing me to load individual ones or every one on a page; extremely annoying at first, but awesome now that I'm used to it
Long URL please: un-shortens URLs (e.g., TinyURL)
Menu Editor: customize menus, including context menu
nURL Suffix: lets you turn off that annoying shift+enter behavior that adds .net
ReminderFox: my schedule tool, replaces such annoyances as Outlook
SkipScreen: skips wait times on sites like RapidShare, very useful for downloading porn copyrighted non-copyrighted 100% legal software, such as Linux ISOs :3
Tamper Data: HTTP header stuff, used infrequently
User Agent Switcher: switch the user agent string, tricking some sites into thinking you're using a different browser
I have other ones, but they're currently disabled. I'm using the Classic Compact theme to maximize screen space (kind of annoying that I sometimes click on a link instead of a tab, but oh well).