The word API is defined as an Application Programming Interface, or rather it is a set of code that allows an external appliation to affect a part of an appliation. If you were to ask what my favorite thing to work on as far as programming or software design is concerned it is in this field. Realistically that is what the Web 2.0 hype is all about. The easy creation of dynamic controls and the ability to access functionality from external applications.

One thing that was made was the tooltip add for World of Warcraft that http://www.wowhead.com/ created. Here is a test of this API:

Merciless Gladiator's Touch of Defeat

The easier you setup an API, the more likely it is going to be used. This may sound obvious, but there are a lot of poor software out there that ignore this fact. There are a lot of things to consider when developing an application and the easy use of APIs should be added to this lengthy list of considerations.