We’re using a cluster of Mac Minis for hosting. They’re running Ubuntu very nicely. I’ve gone on about them before (maybe not on here), but we just got a couple of new ones that use Core2 Duo rather than the older Core Duo models, and wow are they good. How good? Well, for CPU intensive stuff, we’re seeing (per core) over double the performance of our Dell PowerEdge 1850 which sports two dual-core 3.0GHz Xeons (and these are the slower 1.8GHz models!). That’s not bad for <20% of the cost to buy. They are unfeasibly small and really quite cute, plus they only pull about 120W (peak - 65W at idle!), so hosting them is dirt cheap (It'll cost you less than empty rack space - just ask Mythic Beasts whom we highly recommend, and they have minor niggles of the platform under control). Sure the disks are not too fast, and there’s no internal RAID, but who needs RAID when you can have RAIC instead?