Last App is a new and cool application available in Cydia (yes, I’m back into the jailbreaking world) which allows you to switch back and forth between the app that’s running on your iPhone and the one that was previously running.
Just assign an activation method in the Activator settings (mine is “double tap on status bar”) and Last app will immediately switch to the “last application” that was running before you opened another one. You can also choose to background the running app before switching.
I find myself using it a lot. It’s fast, useful and free.