Couchsurfing - A site that allows people with a couch to offer it up to people looking for one to crash on. A great way to save money by not paying for a hotel or a tour guide.
Meetup - Find out what groups are meeting up for various events. It's an open invite to anyone so it's a wonderful way to make some friends in your area or simply find something to do for the day.
Geocaching - Hunt for little plastic containers in the forest using advanced military satellites! It's a fun game that gets you exploring outside.
Foursquare - A check-in site that lets you see where your friends are and let them know where you are. A little stalkerish but you can often get special deals at various locations by checking in. You can view my profile here.
Bookcrossing - Read a book, slap a sticker on it, leave it somewhere, and watch where it goes!