I had no idea Foursquare's API had matured so much, loads of opportunities to build interesting things. This write up has some great insights into some of the challenges of implementing against it.
Yeah, the connected apps thing really is a joy to integrate with. They've had the push API that sends you checkins for a while but they only added the in-app integration points a couple of weeks ago, and it's those that make the whole thing really interesting.
