Android: While Android allows you to run multiple apps in the background, it doesn't have the best method of switching between apps quickly. Dock4Droid brings a swipable dock, not unlike iOS' fast app switching, to your Android phone.
I've never been a fan of the "Recent Apps" menu you get when you hold down the Home button on your phone. It only gives you eight apps, and only gives you the most recent ones you've used. Dock4Droid is one of my favourite fast app switchers I've seen yet: it sits at the bottom of your screen in the form of a small glowing line, and when you tap it, you can swipe through all your currently running apps. You can also tell your favourite apps — things you often need to launch quickly from another app's screen, for example — to always appear on the dock. And, of course, the dock's look is customisable.
It's one of many different app launchers out there, but there's no learning curve and it doesn't get in your way too often. If you accidentally invoke the dock when you're trying to tap near the bottom of the screen, you can just swipe your finger up to close it — though it doesn't happen that often to me.
Dock4Droid is a $0.95 download for Android devices, though there's a limited free version for you to try before you buy. Once you decide you like it, you can download the $0.95 licence key from the Android Market.