Going back to the early 90's there was Xi Graphics Inc that specialized in creating high-performance X Servers and graphics drivers for Linux/UNIX. Their proprietary Accelerated-X product was compliant against X11R6.4 and was licensed to a range of major companies, universities, and individuals for its features and performance. They also developed their own in-house graphics drivers for different hardware (namely early ATI hardware), which they claimed to be the fastest.
Among the advertised features for their products was hardware-accelerated support for multiple displays / stretched displays, support for IBM AIX, SPARC support, and "Our ATI graphics support has been the fastest on UNIX/Linux for years. No kidding."
Their drivers supported hardware like the ATI R300 GPUs and earlier, the 3Dlabs Wildcat and Permedia graphics cards, and old Matrox and S3/VIA products.
It's been several years since hearing anything about Xi Graphics but yesterday a Phoronix reader wrote in about Xi, which brought back memories.
At the Xi Graphics web-site, their outdated web-site says they ceased developing graphics drivers and licensing its software products for the UNIX/Linux market on a per-computer system basis. All they do now is license their "Accelerated-X" implementation to organizations that are capable of writing their own DDX drivers for their hardware. So you can get their proprietary X Server, but you need to write your own hardware drivers for the platform.
However, it's unknown if they still even do the Accelerated-X licensing. With the advancements of open-source graphics drivers in recent years, the binary AMD / NVIDIA Linux drivers continuing to advance, and the X.Org Server slowly but surely advancing, Xi Graphics' products have lost their relevance since the XFree86 days.