KDE 3.x is stable and has been running on Mac OS X for quite a while. Unlike KDE 4, however, they require an X11 server to operate.
KDE 3.5.6 is currently available in Fink stable. To install the basic desktop, type (in a console window):
fink selfupdate-rsync fink install kdebase3-unified
KDE 3.5.9 is in Fink unstable. First, enable unstable following the instructions in the Fink FAQ, then for the complete KDE desktop, type (in a console window):
fink selfupdate-rsync fink install bundle-kde-ssl
If for some reason you can't use
Note that KDE itself on Fink is very solid, it just hasn't been moved in its entirety to Fink stable because of issues moving a few of it's dependencies to stable. It is recommended that you install the latest KDE from Fink unstable.
KDE 3.5.8 is in MacPorts. To install the complete KDE desktop, type (in a console window):
sudo port install kde