I am building a raspberry pi cluster. I am looking for a multi-head micro-usb power cable to power them.
Does such a beast exist? Or can I make one myself?
You can definitely make you own. You can either cut up and existing micro usb cable, and solder the red and black (not always though) to a central (thicker) cable.
You can also buy the plugs themselves
Some people have powered the Pi though the GPIO header. The downside to this is that you bypass the polyfuse and the Transient voltage suppressor diode. So you will lose some protection.
your best bet is to use a cople surge protectors like these http://www.govtechdepot.com/555j-6out-2-5ft-cord-reg-plg-be106000-2-5/?ref=lexity&_vs=google&_vm=productsearch&gclid=CjwKEAjwgMieBRCB3bqB94e9lD4SJABW3sTNsvtAD_2prn4Rri_SPoeJ4iQ0cz2uc-yxrVzAnvjTDBoCcCbw_wcB cheap powersupplys http://www.everbuying.com/product176948.html?currency=USD&gclid=CjwKEAjwgMieBRCB3bqB94e9lD4SJABW3sTN3HadLlzJZUbtrzV7rA99Kdi_1mw8WbSfE9f0lsUNuhoCLQDw_wcB and short micro usb cables