Solaris operating system originally developed by Sun Microsystems and then Oracle. It is known for DTrace, ZFS, Time Slider, running on x86 and SPARC (32 and 64-bit for both), servers and work stations, and being very scalable (especially on SPARC). It is programmed in C and C++ from a mixed set of proprietary and non-proprietary code. The kernel is entirely proprietary but routinely leaked via BitTorrent (much to the dismay of Oracle management. Note however, no one can legally use this leaked code). Oracle's industry partners can apply through legitimate channels to gain access to Solaris source code. Solaris is POSIX and maintains compliance with the Single Unix Specification.
SunOS 1 to 4.1 are a BSD based and only incidentally related to Solaris. AT&T and Sun cooperated in merging BSD, UNIX System V, and Xenix in to what became UNIX System V Release 4. Solaris 2 for SPARC was based on this project. But initially it was called SunOS 5. SunOS 4.1 was retroactively named Solaris 1 (Solaris was not released in 1990, SunOS wasn't even renamed Solaris 1 until 1992). Sun Solaris 2 superseded SunOS in 1993-09 when SunOS support was discontinued. Internally, core components of Solaris continued to be called SunOS (SunOS 5.11 is Solaris 11). Solaris 2 used the OpenWindows desktop that supported SunView, NeWS, and X Window System applications. Solaris can start an X application on one computer while being viewed from another. The application can even be transferred to the second system while it continues to run. Solaris 2.1 was ported to x86 and designed to compete with Microsoft Windows in the Desktop market. It included the Wabi emulator that allowed installing 16-bit Windows (3.x and Workgroups) to run applications on the Solaris desktop (even on SPARC). Solaris 2.5.1 was ported to PowerPC but this feature was dropped from Solaris 6. Solaris 10 introduced the Solaris Containers for Linux Applications feature; running Linux binaries on Solaris (initial compatibility started with Solaris 9 but Sun wasn't bragging about it yet, you know how emulators are). Solaris 10 became OpenSolaris 2005-06 with code available under the Open Source non-free CDDL license. Since some components were still proprietary (Sun was not the copyright holder if these), the Illumos project was created to make a 100% open OS based on Solaris. OpenSolaris also ran on PowerPC at launch. OpenIndiana was a server security specific effort under Illumos. SchilliX was a live-CD distro of OpenSolaris. MartUX was a SPARC platform live-CD then Live-DVD. MartUX was renamed OpenSXCE when x86 was added and after Oracle purchased Sun. MilaX is a 90mb distro of OpenSolaris. Jaris OS is a installable live-DVD with wine for Solaris. Dyson is Debian GNU/Linux running on the Illumos kernel (take *that* SCLA!). SmartOS is virtualization specialist Illumos. Nexenta OS is Ubuntu running on the OpenSolaris kernel. StormOS is Nexenta OS running XFCE (a lightweight Nexenta). Oracle purchased Sun Microsystems, halted public updates to OpenSolaris, and renamed it Oracle Solaris in 2010 (Solaris 11). After many licensing changes, Oracle Solaris can now be downloaded and used for free for an indefinite length of time for certain activities (using as a development platform, commercial use, and "production" use are forbidden). Educational institutions have additional rights such as letting the whole class use Solaris for free if they are earning a degree (high-school and up). So simply gaming on Solaris is free. "Point releases" are free but up-to-date patches and updates (urgent security fixes) require purchasing a support contract.