Although Steve Jobs didn't mention it in his Macworld keynote yesterday, Apple announced its own implementation of the X Window System for OS X that runs alongside Aqua: X11.
X11 is based on XFree86 and is integrated with Quartz, giving it a powerful advantage over similar projects like OroborOSX and XDarwin. X11 is easy to install, launches quickly and looks beautiful. Windows are interlaced with Aqua and can even be minimized to the dock.
Several X11 packages are available at OpenDarwin including GTK+, Qt, and The GIMP.