GameGame - the game that is also a prototyping tool.
In GameGame, players compete in designing games. Players collect and trade cards in order to create a complete game design. In between, one player gets to play a game publisher, while the other players try to sell their game concepts to her. In the end, the best game design is decided in a vote. Let the best game win!
You can also just browse though the cards to try to get ideas. Or pull out a couple of cards and think about how you would make them into a game.
Available for free from: http://gamegame.blogs.com/
I have pictures posted on this page of the cards:
BGG Game Page: http://www.boardgamegeek.com/boardgame/20684
Side note:
I have an auction going on with the "Ultimate game designer's package" available with lots of parts for prototyping. If you win this item, then I will throw in a free copy of GameGame (printed out on 110llb cardstock) with the rules!
http://www.boardgamegeek.com/geeklist/45163/item/1028317#item1028317
If you aren't a member of BGG, just email your bid: Playtester [AT] verizon.net
I do have permission from the designer of GameGame to give away copies of his game.