BlueMaemo is a free application that converts your N900 into a host of bluetooth peripherals including a keyboard, mouse, media remote, gamepad, presentation remote and even a PS3 controller. The application is available via the application manager under the System head if you have the extras-devel repository enabled. If not, click here for instructions on how to do so.
Maemo Italia has a video of the application in action, connected to a PS3. Unfortunately, the video is in Italian but you should still be able to make out most of what is happening during the entire process. Please note that BlueMaemo is still in the extras-devel repository, this means that the application is still in alpha and unstable and may hang/crash your device.