I like going through amateur/indie game communities built around game engines, because you can find some gems in there.
AGS: http://www.adventuregamestudio.co.uk/site/games/ BYOND: http://www.byond.com/games/ Ren'Py: http://games.renpy.org/
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?
If you are not opposed to text-based games, I highly recommend checking out Twine-based games - howling dogs is a great example. Also, generally speaking the Ludum Dare competitions have quite a few excellent entries worth investigating every year.
I hadn't heard of twine; I'll have to check them out. That makes me think of the z-machine and Frotz http://frotz.sourceforge.net/
Oh, another suggestion (though not game-engine based) - the ongoing Live Free Play Hard series on Rock Paper Shotgun. It's great.