Recently bought an used proliant server computer (HP Proliant ML-350 G5) for no more than a namely price, just to get rid of the "trash". These '08 things are getting thrown away from datacenters/companies all the time.
They have xeon quadcore processors and usually lot's of eec ram + raid drives. Very powerful and stable machines but with very limited graphical applications (only those tiny pcie4x slots -> no decent gpu cards available) Excellent for rt audio computation with usb sound interface.
Ubuntu Studio installed flawlessly with jack, ardour and pd running like a dream.
Eats lots of power and a bit noisy though.