Sigh... uninformed people acting like they are posting factual information...
System Preferences > Trackpad > More Gestures > App Exposé
The fact that this is only accessible through a gesture is ridiculous. How are you suppose to do it with a regular mouse and keyboard? The "Magic Trackpad" is ridiculous. It doesn't even work like the Macbook's trackpad, you can't tap to click so switching between the two is confusing at best. Its is also no replacement for Snow Leopard's App Expose.
Studies have been done and gestures are a step backwards in user interfaces, especially when they are they only way of doing things or they are complicated and awkward, which most of the new ones in Lion are. At least the ones before lion felt intuitive and natural.