1. DVR - Expanding on DewChugr's recommendation, this is amazing if you actually like TV. I took an old PC, installed Vista Home Premium and a TV tuner on it, and now have a full blown DVR. I also have a useful app called webguide (free), which allows streaming/recording scheduling through my phone. If you have an Xbox 360, you can sync the two and stream video,etc.
There is even some sexy slimline cases for the full media center effect.
2. Make your own FTP/NAS server. I haven't set one up myself, but this would be perfect for Home and site/domain backups. Always running, redundant storage, and peace of mind. Just set up a cron job and your life just became easier.
Check our
FreeNAS, and for the vaginas there is Windows Home Server.
3. Donate for a tax writeoff (fuck paying in), or sell it. I wouldn't try and give it to a family member, unless you want to be the 'tech guy' for the life of the PC.