I've been recently doing some research around the Desktop Composition Engine (also known as the Desktop Windows Manager or DWM). It allows Longhorn to display Windows shell using cool transition effects, transparency and 3D. These effects are also known as "Aero Glass". Last August, a registry hack was discovered by a member of the AeroExperience boards to enable these features in the WinHEC 4047 version of Longhorn (http://aeroxp.net/board/index.php?showtopic=79)
There have been many attempts to create usable interfaces in 3D. Many of them are listed on the following page: https://lg3d-core.dev.java.net/lg3d-related-technologies.html
Unfortunately, these Aero effects will only be available on the upcoming Longhorn Client. Contrary to popular belief, the November 2004 Avalon CTP does not allow Windows XP or Windows 2003 to natively mimic these effects. Joe Beda explained the reason for this in his blog - downlevel releases will not change any of the native Win32 binaries (http://www.eightypercent.net/Archive/2004/08/30.html). The Avalon CTP simply provides a new API to code against - 3D and transparency effects can be achieved within an app but not within the shell.
If you want Aero-like 3D capabilities on Windows XP, I would highly recommend trying out Sphere XP (http://www.hamar.sk/sphere/). As a power user, I typically multitask with dozens of Windows open at the same time. Sphere XP allow you to rotate, resize and position windows on a z-axis. Rather than try to explain the features to you, check out this page for several compelling screenshots: http://www.hamar.sk/sphere/sphereshots.htm
* Keep in mind that Sphere XP is beta software - don't install it on a production machine.
To get opaque windows on Windows XP, you can try out a commerical product such as Actual Transparent Window (http://www.actualtools.com/transparentwindow/), WindowFX (http://www.stardock.com/products/windowfx/) or Y'z Shadow (http://www2.ueda.ne.jp/~higuchi/htm/Download-e.htm).