This software project aims to create an open-source version of X.org X Window System for compatibility with Mac OS X operating system.
Please be aware that Leopard's X11 package needs to be previously installed for Xquartz to work. If you've previously uninstalled it or deselected it during your OS installation, you'll need to reinstall it from the installation DVD.
It's worth noting that Apple has included some of the work done in the Xquartz project in their 10.5.2 update, and they will likely include further changes in possible future updates of 10.5.x. That said, it's still suggested that you install the latest XQuartz release, especially after updating to 10.5.2 (and any future 10.5.x updates).
It's worth noting that 10.5.2's version of X11 is somewhere between the 2.1.1 and 2.1.2 packages released through this site. Additionally, the launchd mechanism to start X11 in 10.5.2 is unique to Apple's 10.5.2 and never appeared in a release from this site in its current form (though it was essentially integrated into xinit for 2.1.2).
While installing versions prior to X11 2.1.2 onto OS-X 10.5.2 may work, it is not supported, so it's advised to stick with the latest XQuartz release.
Version 2.4.1 Alpha 3 / 2.4.0: N/A