In the past there was OpenSolaris which is the open source version of Solaris from Sun Microsystems. After Oracle bought Sun they discontinued the development of OpenSolaris, so enthusiasts forked it into OpenIndiana and later illumos. You can check them out
However if you don't care about the source code then the latest official Solaris 11 is still free for "developing, testing, prototyping and demonstrating your applications" as per its license. Oracle also provides ready-made VMs for use