OroborOSX is a complete X11 environment for Mac OS X based on the Oroborus window manager and XDarwin running XFree86 in rootless mode, written by Adrian Umpleby.
The interface is pretty smoothly integrated with Aqua: it comes with themes that match Aqua's blue and graphite themes; windows belonging to Aqua and to XDarwin can be interleaved (something even Apple's Classic environment doesn't do); XDarwin windows can be iconified into the dock; the latest version even comes bundled with XDarwin, so you don't have to go to the trouble of installing it separately.
It also comes with a template script for making X11 applications double-clickable or launchable from OroborOSX's menu. Modifying the menu can get pretty tedious, because the order of items is based on the modification dates of the underlying scripts.
I have found it much easier to use with rootless XDarwin than any window manager that has edge-scrolling virtual desktops, a root menu, or a button-based launcher, but I wouldn't want to set up something like this as a complete desktop environment outside of Mac OS X.
The OroborOSX web page is at http://julia.et.ic.ac.uk/adrian/software/oroborosx/.