If life was Super Mario World, Twitter just powered up. FourSquare is an iPhone app based on the Twitter code that along with status updates, also allows you to broadcast your location to friends and lets you find them when you're in the same place.
It's an invention from the creators of Dodgeball, a Twitter precursor that got the boot back in 2005, but unlike Dodgeball, FourSquare uses GPS. Although there is only currently an app for the iPhone, you can use it on any phone with a mobile browser, and apps for Android and BlackBerry are in the pipeline.
You can add Twitter friends and post updates to Twitter, so it's really like a Twitter enhancer. There's also an RPG-ish element to it - each time you check in with the service, you get points; interesting check-ins get you badges; more points allow you unlock FourSquare features. We love it already.