StumbleUpon.com (bought by MS), is a site that helps you find interesting websites based on what you are looking at. Kind of a social network...recommends new sites as you "stumble" upon them. Has a rating function and a bunch of community characteristics to help you find things that might be interesting for you.

del.icio.us basically store your "favorites" for you on the web. So you can use any computer, log in, and have access. Very handy. It also has a discovery function like StumbleUpon.

I don't use either of them too much, but I do like some of the music and book communities to find artists/authors that I might like based on my purchases or profile. Amazon does a good job with this.

fduvall