with wallpaper! very much so.
on one monitor i have a wallpaper i picked out.
on the 2nd monitor the wallpaper switches every five seconds, going through a folder of images i have. it fades between the images - and this is a feature built into the os!
also
[link] offers the mac equivalent of windowblinds for skinning the os.
and unlike windows, you can change EVERY icon simply by right clicking on it, selecting "get info" and clicking on the icon. then you simply copy and paste the icon you want on top of it. to remove it, just cut (ctrl+c) again to take it away. also there's a program called candybar that will apply entire sets of icons to all icons.
anything else?
cheers