I tried using compositing on X-Window (with Kompmgr, of course), and it is amazing. However, it is still in the developmental stages and isn’t very well-known.
It features window transparency, shadows, fading in/out for everything, and more. I’m beginning to see why Windows Vista has nothing innovative. Microsoft showed off how the little icons would reflect the file’s contents – KDE and GNOME have had this for forever for text files. Vista will have fancy see-through windows and stuff – compositing in X does this just fine (kinda slow without a good GFX card though). It seems the only new thing Vista will have is the nice little start button.