There is a nice long preview of Beryl 0.2.0 you can read here.
Beryl is a 3D compositing engine for Linux desktops. It can use 3D acceleration and provides lots of nice effects, including the new ones available in Windows Vista. There are lots of screenshots there showing the features running on Kubuntu. It probably isn't as polished (yet) as the commercial offerings but it is not like Windows Vista demanding a substantially more powerful computer in order to see these effects.
Even at the end of the article there is a nice walkthrough on how to install the package.