This desktop ain't big enough for the both of us

Thanks to Fink, you can run KDE on Mac OS X. See here for details. One half of me loves the idea, the other finds it somewhat less than sane, mainly due to the battle of the taskbars.

Slightly more usefully comes rdesktop, an easy, tiny and speedy way to get a remote Win2K/XP desktop up on X. "Unlike Citrix ICA, no server extensions are required." (By X, I mean the X Window System, though OS X is also covered.)

(For completeness I should add that there's also a KDE for Win32 project that uses Cygwin, but it looks even less desktop friendly. That may be more to do with Cygwin's X server not doing window-per-client yet, which would enable each X app to have its own Win32 window, rather than the whole desktop being wrapped in one big window. There are other X servers for Windows which do have that feature, but I think they all cost a bit. Haven't fully checked.)