StumbleUpon

Listed as one of PC World's 100 Best Products of the Year, StumbleUpon is a Firefox extension which let's you channel-surf recommended websites based on your pre-selected interests. The tool allows you to collaborate with other users and rate any webpage you visit.

Since using the Stumble! button, I've discovered many why-didn't-I-know-this-existed sites. And because Stumble can remember which sites you liked, there's no need to flood your bookmarks. Warning: StumbleUpon is highly addictive!