After MLP:Online was shut down last year most of us expected the developers to give up on fangame development altogether or move to commercial development, but apparently they've decided to make another fangame codenamed Project Cygnus. Unfortunately we don't know much about it other than the fact that it is an action game, but in the meantime they have decided to ease the weight by releasing MLP:Online's source code (stripped of all resources). They are no longer supporting it, but if you want to use it for your own fangame: go ahead! It should work quite well for any RPGs that you plan on making, and if you tinker around with it a bit you'll probably be able to compile your game for Mac OS X and Linux too.

What are you waiting for? Grab the source code from Bitbucket, and it'd probably be a good idea to pick up the tools as well.