The Binding of Isaac trailer: http://www.youtube.com/watch?v=9au_e0im0aM
Steam link: http://store.steampowered.com/app/113200/
This is an indie game designed by Edmund McMillen (art) and Florian Himsl (code). Its meticulous balance, precise controls, complex interaction of numerous mechanics, ridiculous difficulty curve, as well as randomization makes this an engaging, rewarding, as well as endlessly replayable game.
The game boasts a thriving community (http://www.reddit.com/r/bindingofisaac/) and a speedrun league called BoILeR. To see a work so finely crafted and celebrated coming from the efforts of just two people is inspiring. Indie games have been in an upward trend in the recent years, but this is the game that showed that the tools for creating games have advanced to the point that each individual has the ability to impact the medium in a significant way, and those tools are still being improved