Open Source and Free Software

Aqua is more than just a window manager - it's the entire windowing system, more akin to X11 itself than a particular WM or desktop environment. It's possible to install Apple's X11 and from there Enlightenment, or whatever you fancy...but doing that pretty much renders the OS useless. It won't run apps designed for OS X (Carbon or Cocoa), only things that have been built to run under X11 in the first place. Literally nothing that runs on OS X will run under X11; it's not a Mac anymore but rather a Unix box.

If you still want to go for it, instructions for 10.4 are here with issues on 10.5 discussed here, or you could check out PureDarwin - they're doing essentially what you're looking at by packaging the open-source stuff from Apple. They also have MacPorts up and running so you have a decent selection of X11 software.