Here's my entry for the Game In A Day contest, 2009: Gears of War.
It's a turn-based tactics game - you have a bunch of robots, your foe (Either an AI or another human player) has a bunch of robots, and you would both rather like it if the other's robots all vanished.
I'm actually reasonably happy with the quality of the result, given the timeframe. The graphics are utterly awful, as you might expect from me, and the UI is a little clunky - but the gameplay is, I think, actually pretty good, and the AI manages to be semi-competent, in that it's actually trying to hurt you and not doing random stuff (Actually, my first cut at AI for it just had the AI moving 'bots randomly and then firing at any enemies in range...)
I might actually continue the game in the future - probably with some rewriting of code and better graphics, but the same game, at least.