Sun Microsystems' most recent release of the Solaris OS, not including OpenSolaris. It was released in 2005, and runs on 64-bit SPARC systems and 32-bit or 64-bit x86 and AMD64 systems. It omits support for the earlier 32-bit SPARC systems, and also for the UltraSPARC-I CPU, which means that the Sun Ultra 2 is the earliest system it will run on.
Among the highlights of the new release are Zones, a feature that allows the system to run multiple, mostly-independent installations of the OS. Zones are excellent for servers, providing more isolation than a simple chroot jail, without the overhead of an entire virtual machine. It also includes ZFS, a high-performance filesystem and volume manager capable of handling unreasonably large maximum filesystem and file sizes, OpenGL, software RAID (like the old SDS), Role-based access control and a much-improved userland over previous versions. Both the CLI and the GUI have received much-needed facelifts. Most of the standard Sun UNIX utilities, known for their spartan interfaces, have received some of the most useful GNU options, such as the -h (for human-readable) flag. While CDE is still included, the new default desktop environment is the Java Desktop System, which is based on GNOME and actually has rather little to do with Java (although it includes a few utilities written in it). Also new to this release is the Service Management Framework, a new way of managing system services and daemons. This new system is easier for an administrator to work with than the older /etc/init.d method (which is still supported) - but, in my opinion, it's less transparent and discoverable. On x86, you have the option of using the older Xsun X server, or the newer Xorg. Xorg is nearly always the better choice.
The bletcherous and rather defective Web Start installer used by Solaris 8 is gone, replaced by a straightforward, if somewhat slow, system that uses Java and CDE. (There is still something called Web Start, but it has nothing to do with the barfulous monstrosity that was foisted on unsuspecting admins by Solaris 8). Install-time customization is a bit limited, but autoconfiguration, especially on SPARC systems, works well.
Hardware support on x86, while still less than that of Linux or FreeBSD, is much improved from previous releases, including many more device drivers particularly for ethernet and graphics hardware. Also, nVidia now provides a driver for their GeForce and Quadro-series cards under Solaris 10 x86 (but not SPARC, at present).
All in all, Solaris 10 is the most capable and featureful Solaris release yet.