I have one RPi for logging temperatures in my apartment: http://pi.tafkas.net/temperatures/

And another one is monitoring solar panels on the roof of a house: http://solarpi.tafkas.net

I'm using a couple right now to demo using IBM Watson services in real world (physical/IoT) applications - you can expect a blog post on that in a few weeks. (I work on the Watson team at IBM)

I also have one set up as an XBMC media center for streaming netflix and such.

I've set quite a few up as airprint servers for people. It's great to get a few more years out of an old laserjet.