Apple provides X11, an X Windows server for OS X. They also provide quartz-wm, a window manager that gives X11 applications the Aqua windows look. However, that's not the way that most UNIX applications were designed to run, particularly multi-window applications like the Gimp.
There are better, minimalist window managers available, which have lots of added benefits including virtual desktops, launch menus, windowshading and so on. As many applications use the Gnome Toolkit (GTK) for their GUI, it's easy to apply themes to the applications as well.
It takes a while to work out how to set things up to work well, so here are a few pointers.
Screenshot 1. X11 running Gimp in a root window (set in X11's preferences), using Blackbox as a window manager and the Shade theme. BBpager provides the virtual desktops, running in the slit. Everything is set to auto-hide, so it's basically a blank screen on startup. Blackbox is set to maximise windows to the full screen size, and to use Sloppy Focus (windows focus when the mouse moves over them) and Click Raise (windows only raise to the front when clicked on). Quartz-wm is running as a proxy so you can still copy (Apple-C) and paste (middle mouse button or Option-click) between X11 and OS X.
The end of .xinitrc looks like this:
/usr/X11R6/bin/quartz-wm --only-proxy &
/usr/local/bin/bbpager -w &
Screenshot 2. Aterm ('aterm -tr -trsb -sh 60 -bg black -fg white -sr -cr green -fn lucidasanstypewriter-12 -fb lucidasanstypewriter-12 -fade 60') running cplay, an ncurses-based front-end to madplay (for mp3s) and ogg123 using esound as an output device (for oggs).
These were compiled and installed natively from source in Panther, without using Fink. Gaim works too, as does Nicotine.