I am currently developing the game Klimakonflikt with 5 other guys. It is a Retro arcade game, Pac-man style, with nice 2D Graphics and music. Here is a screenshot from the game:
The current release, documentation and source code can be downloaded from here http://klimakonflikt.codeplex.com
The game is inherently for 2 players but we are working on the single player edition. The AI in the current release is pretty daft, but in the next release it will be much improved. Also we are working on a WPF leveleditor, powerups and other fun stuff.
One of my co-developers Jakob has released a blog about the game and XNA development in general at http://xnafan.net