Dodgeball. Basically, you tell the system where you are by text messaging it from your phone. It then tells all of your "friends" where you are. So, you tell it you're at the bar, and it sends an electronic summons to all of your friends to come to that bar. I wonder if anyone actually uses this. Once we all have GPS on our phones, it will be a lot more interesting.
Delicious. Social bookmarking. This I might actually use. Basically, you share your bookmarks and and can look at other people's bookmarks. There are RSS feeds and various other features. Similar concept as StumbleUpon, sort of.