Wasn't there a thread about this a while back?

Anyway, I don't really bother with themes, though it might be worth a look. Extensions I use all the time:

- Greasemonkey
- TabMix Plus
- Flashblock
- Adblock
- Video downloader (though it seems to be broken for youtube, so I resort to another site)

For work purposes, add:
- Firebug
- IE Tab

Other stuff I'd like to try:
- There's one that has a language switcher menu, and even does inline translations for foreign pages.

Extension I've tried, enjoyed, but haven't had the time/patience to learn to properly use:
- Vimperator (too bad, too - I love vim)