You can run fluxbox, FVWM, and Gnome using darwinports but it is not something I would recommend. If you want to use X11 then boot to Linux. Aqua is not an X11 app. I would say it is closer to X11 than it is to Windows but it is a proprieatary app sitting on top of Darwin.
OS X = Distro (Redhat)
Darwin = kernel (Linux)
Finder = desktop (yes, the desktop in OS X is generated by finder)
Aqua = windows interface (X11+Windows Manager)
The look of Aqua can be customied by haxies but in general it's behavior can only be extended, not replaced. Apple is a company looking to sell a product. They want consistency of user experience not 33 flavors and then some. What you are asking for is almost anathema to the user interface guidelines. (EDIT: I actually think competition in the desktop and window manager environment is a good thing, but Apple is a company. It makes no sense for them to compete with themselves.
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There is quite a bit of free software out for the Mac. While not in the quantities available for Linux in general the Mac freeware is more usable. (Sweeping generalization I know.)
EDIT: Gimp works the same as in Linux, or at least as far as the basic stuff I do with graphics.