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).
***Note (11/22/2011):
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).