For me it has got to be Retroarch/Libretro (or as I've taken to call it etc.).

Retoarch is a frontend for Libretro which is an API that emulator developers can use so that I as the user don't have to worry about configuring each emulator (and some other stuff).
It's a godsend piece of software that makes it possible for someone like me to really just enjoy my (but but expanding) library of old games (that I can only dream of playing on original hardware)
Also, it's multiplatform!

I also tagged it as wk119 since this is my school setup