I like going through amateur/indie game communities built around game engines, because you can find some gems in there.
I usually go and check out AGS every so often to find interesting adventure games. There's quite a few good adventure games, like the Yahtzee series (yes, the same Yahtzee from ZeroPunctuation, before he became internet famous).
With BYOND, I mostly have played SpaceStation13 and Nestalgia.
I hadn't heard about Ren'Py until just now, and I'll definitely be checking those out later.
Does anyone else have any other game engines that have a large community built around them? Any game gems to share?