A few goods ones I use/have used:
WOT (Web of Trust) - Tells you user ratings for websites about trustworthiness, safety, etc.
EyeDropper - Allows you to pick a color from a webpage (kind of buggy)
Cooliris - Allows you to scroll through Facebook photos and Google Image search results on a large 3D wall
Feedly - Manage RSS feeds
StumbleUpon Toolbar - Jump around the web to random websites and images depending on categories
TinEye Reverse Image Search - Allows you to find where images have been uploaded before, useful for exposing phonies and finding the source of images
Google Dictionary - Double-click a word to show a little bubble with the definition
Smooth Scroll - Makes scrolling smoother