NEW:
Sun Microsystems's SOLARIS 10 is free too!
And there is a version for the x86 (INTEL, AMD).
hardware compablity and specs:
http://www.sun.com/bigadmin/hcl/data/sol/
http://www.sun.com/software/solaris/specs.jsp
download free from Sun Micro:
http://www.sun.com/software/solaris/get.jsp