Some extensions I use:

ForecastFox - Pretty cool, I have the current weather, and the forcast for the next few days on the bottom bar of the browser, out of the way - but handy for a quick forcast.

BugMeNot - Some websites make you create a login & password just to view something on their site - something which you may only want to spend 5 minutes looking at and never come back. BugMeNot will search through it's records and fill in the userID & Password for sites like this if it has it so you don't have to.

StumbleUpon - If you are bored, this is pretty cool - find random internet sites - rated by other stumbleupon users.

I also have DictionarySearch & Translate - but never really used it other then loading them.