Most programming is done on an existing framework or platform, and using various APIs which make your programming job much more convenient.
It can either be a superbly designed set of APIs, or a nice framework which you managed to get really cool things done really fast.
All answers should give concrete examples of why that framework is so awesome, or examples of cool stuff you did with it.
What existing framework really inspired you to do some awesome work?
I know this is subjective, but I think we can get some cool ideas from this question. Please be easy on the "close" link :) Also - commun-ikified in advance.