My first rig is probably the world's second ugliest miner:
http://noosworx.com/post/2917305636/worlds-second-ugliest-bitcoin-miner

Since then I've added a small 6 GPU cluster (pic says 5 but I've since added another one):
http://yfrog.com/h7i69haj

I buy second-hand mobos and cases from FreeGeek and outfit with new PSUs and second-hand (Craigslist) GPUs.

Current total hashing rate is 2.9G. Total cost so far ~CAD$2K.

Poclbm under Ubuntu 10.10.

((Yes that's a Tor sticker on one. It's also acting as a Tor exit node))