This one is informal, and doesn't have a strong community yet, but it is growing. It is promoted by Adrieal Wallick, here's her GDC talk http://www.gamasutra.com/view/news/227022/Video_Lessons_learned_from_trying_to_make_a_game_a_week.php .
Here are the Guidelines:
- WEEKLY DEADLINE
- SHARE THE GAME, GET FEEDBACK
- TRY TO CHANGE IT UP EACH WEEK
- REFLECT ON WHAT WORKED AND WHAT DID NOT
Ranked in these QuestionsQuestion Ranking
Pro Flexible Schedule
You don't have to work on your game on any particular day(s), therefore you can work when it fits your schedule best.
Pro Informal Rules
Game A Week is more of practice than a contest, therefor you can apply rules to yourself that enhance your experience and creativity.
Pro Short Time Horizon
Having a quick deadline helps focus your effort and also keep your scope in check.