One Game a Month is a website that features the challenge of making a game every month. It currently has 5519 users and 3309 games. All the guidance you get is an optional theme for the game, no pressure, no commitment, just the pure want to produce the game in the space of a month.
I have yet to look into the specifics of the site, but they do have audio files of each months theme back-dated, so this could be a good place to get some ideas for my own version of the challenge.
The site also features various resources and advice sections that could come in handy.