RISE.

Name: One Game a Month
Dates: Can be started any time
Time limit: From the beginning of each month to 96 hours after the end of said month.
Cost: None
Participation: Online only
Website: http://www.onegameamonth.com/
Rules: Begin working on your game at any time during the month. You'll have until the end of the month to upload your game to a file sharing service such as Dropbox or Mediafire and associate it with your account on the website. You'll need to sign in via a Twitter account on the website to create your account. At the end of the month, there is a 96 hour grace period in which you may upload your game. You'll earn experience points and achievements on the website for making your games, as well. There is a 100% optional theme for each month.