Apple provides a complete X11 environment, you can install all the Fink and MacPorts stuff you heart desires. I run some stuff myself that's been ported to OS X but not written for Aqua (e.g. Wireshark).
But as a replacement, no. You categorically, absolutely, unequivocally cannot run OS X apps under X11 - Aqua is the entire end-to-end display system; it's the display server, window manager, 3D system, and everything else all rolled into one. It's actually a pretty elegant modular system (Google up Quartz and CoreImage), but at any rate, the whole package is indispensable for running Cocoa apps. They simply don't make X11 calls.
If you just want to change the look, there may be themes available, but as I'm already late for work you're on your own 
Edit history
Recommendations
0 members have recommended this reply (displayed in chronological order):
= new reply since forum marked as read