One of the best things about Android is the level of customisation it offers. If you like to tinker under the hood, Android offers you more scope for customisation than pretty much any other mobile platform. For most people, however, simply changing the default launcher will be impressive enough.
LauncherPro may be the most popular alternative, but ADW.Launcher is also worth checking out, as it’s every bit as good. It provides up to seven homescreens, allows you to store shortcuts in a dock at the bottom of the screen, and gives you the choice between a side scrolling app drawer (similar to the iPhone or Galaxy S) or one that’s closer to the native Android one. There’s a whole host of other UI tweaks as well.
ADW.Launcher is a serious competitor to LauncherPro. Which you choose will largely come down to personal preference.