KDE

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KDE is great. It's immensely customisable, colourful, functional and friendly. It needs a decent computer to run fast, as with any modern GUI, but all the eye candy is configurable.
To get a working setup, I used Fink (via Fink Commander) to install the base KDE packages (kdebase3 and kdelibs3) and a few optional packages, plus Gaim, The Gimp and Nicotine.
When X11 starts up, it reads your ~/.xinitrc file - if you comment out the "xterm &" line (to stop the terminal loading) and the "exec quartz-wm" line (to avoid using Apple's window manager, which doesn't handle multi-window applications and other oddities well), then add "export KDEWM=kwin" and "/sw/bin/startkde >/tmp/kde.log 2>&1" to the end of the file, starting X11 should load a KDE session automatically. You can choose a theme in the configuration wizard on the first run, and switch off the KDE desktop (Control Center>Desktop>Behavior>Enable Desktop Icons) to avoid covering up the OS X icons. Right-click on the panel to add icons for your new applications, and you're all set. Here's a screenshot, with each application running on its own desktop and the (hideable) OS X dock down the side: