Apple implementation of X Windows; ships with OS X 10.7.
Apple X11 for Mac OS X offers a complete X Window System implementation for running X11-based applications on Mac OS X. Based on the de facto-standard for X11, the open source XFree86 project, X11 for Mac OS X is compatible, fast and fully integrated with Mac OS X. It includes the full X11R6.6 technology including a window server, libraries and basic utilities such as xterm.
What's new in Apple X11
Version 2.6.3: Release notes were unavailable when this listing was updated.