Andrew invented a strategy board-game called Dugi that looks like a fair bit of fun (looks a little like Pente or Go — he bills it as being "as simple as checkers, as strategic as chess). He's released it under a Creative Commons license and invites you to improve on it. Though I'm pretty sure that this isn't the first "open source boardgame," it certainly looks like it would be fun to play!
PLAY: Players take turns moving one piece per turn. The pieces may be moved 'forward' (toward or away from the centre of the board) or 'sideways' (left or right around the ring). The pieces may be moved as far as the player wishes, but must not leave the board on the outside, enter the dead zone, move diagonally or 'jump' a piece in its way.
To capture a piece, you must move your piece so that it completes a 'surround', then remove the captured piece from the board.
Update: Tom sez: How about an entire Zine of paper strategy games that is under a CCL? Countermoves has put out about 5 issues that can be had as a print ready PDF or from a few game stores and conventions when folks get around to printing them off in enough numbers to share. Gurilla Publishing for Gamers. There are a ton of games in the issues that have come out and the zine is just about to spawn an entire CCL game system called the Countermoves Micro Game Engine. Folks will be able to take the rules and create pretty much any micro sized strategy game with it.