Home file server with ZFS - http://breden.org.uk/2008/03/02/a-home-fileserver-using-zfs/
Proxy Server - http://devcentral.f5.com/weblogs/macvittie/archive/2008/10/0...
Map-Reduce fun with Hadoop - http://www.michael-noll.com/wiki/Running_Hadoop_On_Ubuntu_Li...
Donate bandwidth with a Tor Server - http://ubuntu-tutorials.com/2007/08/31/how-to-run-a-tor-serv...
More good ideas - http://www.dailycupoftech.com/2007/09/17/put-that-old-comput...
A Dial-In server - http://www.howtoforge.com/linux_dialin_server