Are we really limited to Pis, or other microcontrollers like Beagels and Arduinos?
If just Pi:
1) HTPC - very simple and common
2) Arcade Machine - again very simple and somewhat common
3) Media Services for a vehicle (audio & video. Can have multiple medias playing to multiple monitors) - pretty easy, though not super common. Had some interesting power design aspects we had to overcome, but wasn't too bad. Phase II is going to be mapping and GPS, but we have a bunch of logistics to work out for that. Also a few other ideas I can't talk to yet because it's for a design that may be marketed if it works out...
4) Home automation - this I'm just really getting started on, but a Pi is going to be at the heart of it, with Arduinos carrying some of the load along the way. Everything from lighting and outlet power control to temperature monitoring, HVAC system scheduling and intelligence, windows and blinds control, HTPC control with scene tie-in...
And a backlog of about 15,000 ideas that I need time to work on