I think using XBMC will be huge. Cheap, low power, and simply point it at a computer/external HDD with a media collection to play all available media.

Also, using it as a gaming machine (see the Quake III video on Raspberry Pi's site) and game emulators for older console games.

I would love to see different sensors being attached for cheap, open-source home automation projects (home heating, turning off lights when no-one is present, monitoring home energy consumption, talking to mobile devices etc.)

Edit: Project Ideas from Raspberry Pi's forum:

http://www.raspberrypi.org/forum/projects-and-collaboration-...

Combined with poerline networking like HomePlug devices, you could drop one behind every TV or monitor in your house and stream video from anywhere.