I'll probably be banned for this, but I think you can actually install the OS on as many computers as you want. Apple wants you to only install it on one computer. They trust that you won't be a phallus and go around installing their OS on millions of computers.
Depends on the licensing agreements - you guys read the question, don't just answer. Apple states that if you buy a Single License version of Mac OS X you can ONLY install it on ONE machine.
If you buy a Family pack Apple states that you can install it on ONLY 5 different machines.
Windows states you have to register it online in order to use it.
Linux... well, some are under the GPL so it all depends on what it states in the licensing agreement.