Yup, one guy modded the OpenXcom to feature the venerable Ultramarines. Of course this a work in progress, it was just begun recently. But it has potential, if only GW would graciously let it live long enough...
http://openxcom.org/forum/index.php/topic,3261.0.html
http://www.openxcom.com/mod/space-marines
They'll probably get a Space Lawyer drop from GW once they find out.