Unfortunately, Solaris is not free as in freedom, but some releases are free as in free beer. All in all, it is a really nice Unix system, stable and very easy to use.
Too bad Solaris x86 didn't make it, but at the small amount of supported hardware, nobody is surprised. Solaris 10 for Opteron may become serious competition for Linux, because the source is open, there are free distributions and once the new filesystem ZFS is released, others will be measured by offering something comparable.