The Accelerated-X server is built on top of the X386 X server that was created by Thomas Roell for the official X Window System distribution. He would later found a company in Colorado named Xi Graphics which still provides the Accelerated-X server.
The XFree86 project was intentionally created as a free alternative to the Accelerated-X server.
A particularly notable feature of the Accelerated-X server is that it supports "overlay mode" on several graphics cards which basically means that old UNIX-programs that are tailored to 256 fixed colors (8bit PseudoColor) can run in parallell with modern applications using 24bit TrueColor.