Gatsby, one of my personal favorites, provides a virtual concierge who algorithmically attempts to introduce you to people in the same venue that share interests based on a few simple questions.
The idea is intriguing, and could really take location-based services to a whole new level by bridging the gap between semi-anonymous lists of check-ins and real-time, in-person social networking.
Gatsby acts as a perfect layer on top of Foursquare to give it something it's missing - a link to like-minded people around you.
My very first experience with Gatsby was like something out of a romantic comedy. I received two messages in quick succession: the first from Gatsby, introducing me to a girl, and the second from the girl. Within a few minutes I found myself invited to an art party happening at exactly that moment... And, I got a new friend out of it! Gatsby, you're a hell of a guy.
Proof of the @meetgatsby concept: The first time I ever answered one of their texts, I met Shorty Awards founder @Gregory at #140conf.
meetgatsby.com is exactly the kind of malleable social graph app @Scobleizer talks about here.