Android has some great advanced features, but if you prefer the simple, beautiful home screen look of iOS, Espier Launcher gives you the best of both worlds by recreating the iOS experience on your Android device -- without Apple and their walled garden.
Espier is actually a very solid launcher for Android. It's smooth, has lots of nice features and does a remarkable job of looking exactly like iOS. You don't have an app drawer, which may be annoying for some, but you get iOS-style icons in a 4x4 grid with a dock at the bottom. If you swipe up on the home screen, you get an iOS-style fast app switcher, where you can close running apps and, if you swipe to the left, even change brightness, Wi-Fi, volume and other quick settings. Even the folders work exactly like they do in iOS: just drag one icon onto another to create a folder, and when you tap on it, it'll pop up in true iOS style. It also has a search function in the same place you'd find Spotlight on iOS, though it didn't work quite as well as we'd have liked, putting apps all the way down at the bottom and filling the results with unimportant things (like tracks on Last.fm, if you have it installed).
Overall, though, if you like the freedom that Android gives you but prefer iOS' somewhat more polished interface, you won't be disappointed by this launcher. I'm pretty stunned at how well it imitates iOS, where many other theme packs and similar apps fail. Although, if you're looking for a slightly more Android-like experience with iOS-like looks, you can't go wrong with the previously mentioned MIUI ROM, either.
Espier Launcher is a free download for Android 1.5 and up.