LÖVE is an unquestionably awesome 2D game framework, which allows rapid game development and prototyping in Lua.
Extremely easy to use and learn.
Very advanced and full of features.
Lua flexibility when figuring out new game combined with C speed when actually playing it.
Very helpful and forebaring community. Very alive too.
If you plan to make 2D game, this is a place where you should start. Period.