StumbleUpon

If you have Firefox and don't have StumbleUpon... Shame on you.

In short, the StumbleUpon extension is a little button you put on your toolbar. You tell it general categories of things you like, press the button, and it takes you to a site that people with similar interests suggested.

I really can't do it justice in words, but it's such a tiny app that you have to try it. A word of warning, though: I have no fewer than 10 friends who've installed it, and every single one of them became completely addicted to it. Give it a shot.