There's probably oodles. Off the top of my head, there's Apple's web page for X11. Except for a spinal tap joke, the whole first section of the page was devoted

There's probably oodles. Off the top of my head, there's Apple's web page for X11. Except for a spinal tap joke, the whole first section of the page was devoted to the acknowledgement that they were building on top of OSS from XFree86.