One of the most important--and too often forgotten--pieces of a successful API is promoting what developers have built. Doing so should be a win-win, as both platform and developer should benefit from third-party apps being discovered by users. Location-sharing service Foursquare just re-vamped its application gallery and now any developer can be included.
Some apps aren't meant for your standard check-in. Mob Zombies "pits you against zombie hordes generated by human activity at real places." Shakedown is built specifically for people waiting in the Shake Shack line in New York City.
When Foursquare launched its API, it started with a simple gallery. It has now been expanded and categorized. And the best part--you can add your own (login required). If you have a Foursquare app, make sure it gets the star treatment, or see the full gallery for inspiration.