A very cool new app in Cydia called LastApp lets you quickly switch between the last two apps running on your iPhone. Think of it as Alt-Tab for the iPhone.
It only works with jailbroken iPhones (obviously). Simply go into Cydia and then choose your activation method via the Activator preference pane in Settings. You can set it do double-tapping the Home button activates, short hold of the Home button, or single press of Home button at SpringBoard. However, my double-tab is preset to launch Camera, so I didn't want to change that. Fortunately, LastApp supports "shaking", so you can shake the iPhone to toggle between two apps. Is that not cool or what? It also supports slide gestures (drag onto screen from below, drag onto screen from bottom-left or bottom-right) and a plethora of other methods you should check out.
Also, with Backgrounder installed, you can optionally set the current application to background upon switching.
Get LastApp for free in Cydia using the BigBoss repository.
P.S. Hat Tip to Rich Tehrani for the tip