Ludum Dare is a 48 hour game dev competition. It has run seventeen times already. If you like experimental gameplay, I can’t recommend it highly enough– check out the top entries for any given competition. Each entry only takes a few minutes to download and play, so you can tear through more new ideas in an hour than you might see in a month of major releases. Just don’t go looking for full-fledged games: these are experiments. After all, they were made in only 48 hours!
In case you’re still on the fence, Enviro Bear 2000 was originally made as an entry into Ludum Dare 16. Yeah. Ludum Dare is that good.
My favorite entry from Ludum Dare 17 is Gaiadi (also the overall winner). It combines tower defense with shooter– and judging from the result, there’s clearly a lot of promise in that concept. There are a lot of directions to take (maybe a Herzog Zwei adaptation?). Five minutes playing Gaiadi will fill your head with game ideas for weeks.