is very new and is essentially designed to help people navigate unfamiliar spaces. You go to, say, Philadelphia, and you don't know where to go for fun, or eat, or stay, and you check FourSquare and get recommendations from friends.
Or your friends check in (if they have it on their phones) and let you know where they are, so you can come on over if you like. Enough check ins and you can become "mayor" of your area (more of a more trusted user, so far as I can tell, but also a user who does more funky and interesting things). Plus you can get badges for doing interesting and offbeat things, but I'm enough of a fogey to probably not qualify for many badges -- I'm just not out late enough at night.
So, does anyone else FourSquare???
I hang out as shrinkingjes