I've noted previously that one of the non-Sun distros of OpenSolaris (Nexenta) is available in a VM. Now, Sun has now quietly added the Solaris Enterprise System Virtual Machine 1.0 to the Sun Download Center (as a free download). This is Solaris 10 Update 1, plus the Studio 11 compilers, plus all the Java Enterprise System software - app server, web server, directory server, etc. - all in a VMware virtual machine. No OS or application install, just download and untar, fire up VMware Player/Server/Workstation (take your pick), and power the VM on.
The VM seems to work pretty well, though there are a few caveats. The virtual disk is IDE, so don't try to use this with ESX or VI3 (which don't support IDE virtual disks). It's also 20GB, so be sure to have plenty of disk space available on the host. Also, this is update 1, not update 2, so it doesn't have the latest patches (or ZFS for that matter). And the VM doesn't have the VMware guest tools installed, though that's easy enough to do if you're running a recent version of Player, Server, or Workstation. Installing guest tools will improve the console UI experience and networking performance (though be sure to fix up the /etc/hostname.* and /etc/dhcp.* files after installing). (If anyone from Sun involved in creating this is reading, I have some suggestions for improvements.)
I assume the various included components each have their own license restrictions, so examine the licenses carefully. Solaris itself has a free right-to-use but you'll need to pay for support.
The README for the VM mentions the availability of a smaller S10U2 VM without the JES and Studio apps, but I haven't been able to find it. A search on sun.com just hits the VM above as well as a bunch of references to the JVM.
Update (3/19/07): there are now four Solaris VMs available from Sun, including S10U3 and Solaris Express (aka Nevada) build 55. VMware tools are pre-installed (at least in the two I downloaded), but the VMs are still using IDE disks so they won't work for ESX/VI users.