Here are some you might try, that I find useful
(in general it's probably better to minimize the number of add-ons/plug-ins, so don't add any you don't really need):
Mostly Crystal theme with small icon setting
Adblock (filters some adverts out on web pages)
CustomizeGoogle
FEBE ( backup profile)
FireFTP (if you need FTP. I have it but still prefer to use the old WS_FTP95 LE though)
FlashGot (helps with downloads)
Free Download Manager Plugin (if you use Free Download Manager. I do.)
Image Zoom (lets you zoom in on pictures. You can set the zooming steps)
Link Checker (if you maintain a web site and want a tool to check that its links are valid)
Live TV Toolbar (My brother just recommended this to me last month. I haven't started using it yet, for lack of time)
NoScript (Stops javascripts unless you explicitly permit them to run. Can be a bit of a nuisance because it pops up so much, but it really helps defend your computer, so worth it to me.)
TorButton (If you use the Tor-Privoxy-Vidalia budle for proxy stuff. I do.)
Sticky Password Autofill Engine (If you want it to help fill in blanks on web pages.)
Edit: Oh and also
GreaseMonkey.
http://www.greasespot.net/
Then after you install it, you can install various scripts FOR it, such as several scripts available to improve the user's experiemnce of this forum, and others.
http://userscripts.org/scripts/search?q=anandtech
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