Fasterfox, not sure how it works, I don't know any of the tech specs on it, it may be the placebo effect for all I know, but as far as I can tell, Fasterfox improves load time dramatically, though like I said, it's possibly a placebo effect, though they give some tech reason for it there.

FasterFox - Improves load time dramatically IMO.

ForecastFox - Tells you the temp in your area (by zip code) right in the bottom pane of your screen, and a mouse-over tells you current conditions, clicking on it can give you hourly, etc. etc. - very helpful tool. Not sure if I'm allowed to contribute to this or not, but those I have installed on all my comps, they come in handy, or at least I think so.

EDIT:
OMG: I just installed tab effect - that is completely awesome - complete rip off of mac's multi-desktop feature, but I'm loving it, thanks for that, man.