A simple, slick GUI wrapper around the Foursquare API. Perhaps you've wondered how I was able to secure the New York City, Player's Club and 53rd/6th Halal Cart mayorships? Well, it wasn't exactly this (rather a cron job running some scripts that leveraged the API from my EC2 instance) but it's still pretty cool. Available in source form or as an OS X app bundle. (sole dependencies are Python 2.5 and PyQt, so building for platforms other than OS X, so long as these tools are supported, should be straightforward).
As of November 2011, I am no longer maintaining this app. Foursquare has completed deprecation of the v1 API (upon which the app relied) and I have no interest in porting to the v2 API (nor is it practically feasible).