You should check out the Game Design Wiki. It started out wanting to be a collection of design patterns for board game design, but not many patterns have actually been written up yet so it's more a collection of "good ideas", including links to other game design essays. It also seems to have branched out into computer game design recently. Unfortunately it's a little difficult to navigate but there's some good stuff buried there. It's also a Wiki, which means you can add (links to) your own essays or otherwise contribute to the content.

There's also a board game design Yahoo Group, although that tends to focus a little too often on the production issues of making a game. There's also the Board Game Designers Forum which I've never really gotten around to exploring because I generally don't like web-only discussion groups.

Sorry if you already knew about these...