SCO (The Santa Cruz Operation) General/UnixWare.
This company current holds rights to System V and if it has it’s way will be turning GNU/Linux into a $700+ per processor licensing nightmare.
See [ http://www.groklaw.net/ ] and [ http://twiki.iwethey.org/twiki/bin/view/Main/General/SCOvsIBM ] for more information.
SCO Website: [ http://www.sco.com/products/linux/ ] [ http://www.sco.com/scosource/ ]
Oh, just in case you want to pay for your Linux installations up front [ http://www.sco.com/scosource/linuxqanda.html ]. Sorry to say, but you have missed the promotional deadline of October 31, 2003.
Remember this; Just say NO to SCO!