I've done a few projects with the Raspberry Pi:
I created a portable web server that made it possible to communicate local status and information to smart phones and tablets--great for a remote command post or similar set up where you need to communicate information to a potentially large, but local, crowd. Used all open-source software. Total cost < $100 and no Internet required to deploy, i.e. it would work in the field and off the grid.
I just finished building a stratux (
http://stratux.me) box and now I watch the planes flying past. This configuration works with dedicated iPad software so I can watch them whether I'm flying or sitting at my desk.
I have all the parts to build a Raspberry PI SDR general-purpose receiver, but I haven't had a chance to hook 'em all together in that configuration.
Some kinda fun!