http://www.serversplus.com/servers/tower_servers/dell_tower_servers/t20-9186
I have done it in the past, and the rebate is pretty easy. They do tend to make it timed so you possibly forget.
The specs are pretty boring, pentium chip, 4GB ECC RAM and a 500GB 7200 hard drive...... However it still is a very capable machine with the added bonus of being able to take any Haswell chip including Xeons. The machine can take a graphics card such as a 750TI and even better is Ubuntu certified. It is basically a reboxed Dell professional CAD motherboard in a poweredge case.
It's quite suitable as a server too I heard!