http://FoursquareDoor.com - The next release of http://apartm.net's venue-monitoring whitelist action service, using the new push notification api, adds custom callbacks, data passthrough, per-venue slow and fast monitoring, and a few other surprises.
Badge Assistant - A map visualization of where your friends have earned the badges you still don't have.
Treasure Hunt (work in progress) - A game where you go find a treasure by going place to place following clues
Foursquare Munchkin (not presenting today) - Battle monsters and level up with your checkins! Kingdom of Loathing meets 4sq: a lightweight, humor-filled RPG.
Foodie - Check-in not just to places you go to, but to menu items (food/drink). Next time you open your app, you can not just see interesting places, but also interesting food/drinks in the vicinity. Live now at http://www.socialtelescope.com/foodie/.
ArduinoSquare - enable an Arduino micro-controller to check-in on foursquare, but the big idea is sending the micro-controller in a low-orbit rocket/balloon to get the NASA badge ;)
BearMode (Not presenting today)- Your personal online trainer, set a fitness goal and forget the rest. Get personal workout plans via TxT msg and voice reminders if you miss a goal. Workout, no excuses!