This app requires a jailbroken iPhone, but it's pretty neat. Multiflow brings Expose-like functionality to iOS 4.0.1. Installation should be familiar to anyone with a jailbroken iPhone. Enter Cydia and search for "Multiflow." It is a paid app, so bring your US$4.99.
Once installed, you'll want to select an activation method. There are several options, like a double press or press-and-hold of the home button, singe or short press of the headset button or sleep button or shake. Once you're set, try it out. Four windows will be displayed at a time representing running apps. Tap anyone to bring it up front, or hit the "X" to quit it entirely.
It's a nifty alternative to Apple's fast app switching. If you've got a jailbroken iPhone and 5 bucks you'd like to part with, check it out.