One thing I like about Android is that it's highly customizable. A good place to start customizing your Android device is the launcher. There are many launchers available on Android market, both paid and free. Below I will show you 4 of the most popular and beautiful launchers plus one that hasn't been released yet.
ADWLauncher EX is one of the most popular launcher replacements on the Market with crazy transitions and UI effects. It supports themes, it is highly customizable and I believe you should give it a try. There are two versions a free stripped down and a paid one.
GO Launcher Ex is the extended version of GO Launcher. It supports customizable home makeover, themes, smooth operation experience as well as various practical utilities.
LauncherPro is a simple, lightweight and really fast launcher. Have a look at the 3D App Drawer feature.
SPB Shell 3D is an eyecandy but rather expensive launcher. You can have a look at it through the video below.
This one hasn't been released yet since it's under development but I liked what I saw on its preview video that I thought of posting it here.
So what is your favorite launcher? For the time being I am quite happy with the Sense launcher of HTC on my HTC Desire.