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There are a few modules available at launch, such as a display, digital camera, GPS, and accelerometer. There are 81 modules under consideration and with the fully hackable nature of the hardware more modules will probably be coming from the public.
The Bug Labs modules are plug and play and are a mobile geek's playground. There are some reduced prices now for early adopters as well.
Garmin has several GPS devices available and the higher end models have integrated MSN Direct service.