With the rollout of Facebook Places, it's clear the Geolocation is the big thing in social. Foursquare has been pining away in that space for a while now, but they aren't just standing still. Their API has been used as the backbone for other services that need a database of places. Instagram is perhaps the most notable start-up making use of the Foursquare API.
According to TechCrunch, Foursquare is in the process of developing a new API for their service that is "1,000 times faster". This new API will also lay the groundwork for some features the be rolled out at a later date. One detail Foursquare has let slip, is that the new API will offer tighter integration with special deals at various businesses. The better, faster, smarter API is not yet available publically, but an email to the Foursquare team could net developers early access for testing purposes.