Del.icio.us is probably the most widely used social bookmarking
site, it has features on it to help you identify and watch what other people with similiar interests are saving as well.
Mag.nolia is another one, not as big but has a loyal following of users and I love their logo!
sites are places like Linkedin and Facebook, there are also smaller,more focused places like Fast Pitch.
Social News Aggregators
are sites like Digg, Reddit, Stumbleupon and the new Small Business News
. We say they're "social" because of their voting functions.