After a bit of a false start, I finally got mine properly up and running with Raspbmc last night, which is a distribution of XBMC for the Pi. I use Plex Server for my media, and the PleXBMC plugin allows me to access my content over the LAN to the server. The coolest feature though has to be the CEC functionality that works out of the box with my Samsung TV. It allows me to use the Samsung TV remote to control the media centre, including scrolling around the menus etc. Very handy to have the one remote.
I also plan to have a play with running Asterisk on it this weekend, like sbiddle posted about as this sounds very cool.
At the moment, I am running mine without a case. It's currently suspended behind my wall mounted LED TV by the cables plugged into it. I have ordered the standard black case from Element14 but it's on back order.
So.....what have you done with your Pi? Or what are you planning to use one for?