If you don't know about Foursquare, you must tell me what rock you've been living under.
I've been a fan of Foursquare longer than any other social website since their launch back in 2009, and I would use it on my Sprint Palm Pro (these were the good ol' times). I remember how plain the app was, and at the time it was just about considered pointless to have. I remember you being able to checkin, become a mayor (which would be easy because no one else was even using the app) and leave one tip. The interface was just blue and green.
Now, in 2012, and after many years of hard work and 20 million more users, the app is something I enjoy using everyday. I can see what all my friends are up to, leave pictures of the revenues or venues I'm at, leave tips so when others are in the location, they can see. I can make to-do lists for all the places I wish to go or follow other users lists as well. Unlocking awesome badges from checkin into places. Earning points. Some businesses use Foursquare to reward people with specials when you check in to their revenue. It has many awesome features, which I won't go into all the little details now, but if you're a Foursquare user, you have the idea. But just like every other social app, it's only fun with friends.
A lot of people now-a-days aren't ones for an app such as Foursquare, due to giving out their location. Such people are still uncomfortable with spreading such information. Just remember, the only people that see your location is your friends. Also, this app isn't really meant for people who don't travel or isn't near a lot of revenues.
Just recently they added a history page, which allows you to go back and view all your past checkins. They also made Foursquare more interface friendly with Facebook (given the option to share on there and Twitter), making the pictures you upload with your checkin more fitting. So I'm really excited about this big update they have coming to the app. Though they haven't said what will be new in this massive update, they just noted the release will be "sometime this week" on both their Twitter and Facebook.
Note: The Foursquare app is available on all Android, iOS, and Blackberry devices.