You don't need a 42u rack. You can run a cluster of Raspberry Pi's hidden in your basement ceiling rafters like me.
> <erno> hm. I've lost a machine.. literally _lost_. it responds to ping, it works completely, I just can't figure out where in my apartment it is.
Heaps of Pis in rafters will quickly turn into a cable spaghetti hell tied into ugly knots.
Introducing the... "RafterRack™" ;)