One of the most frustrating parts of using the iPhone is that it does not allow you to quickly switch between applications. Seriously - my kingdom for a back button. On a PC, we use Alt-Tab constantly and it is incredibly productive. Sadly Apple does not have a way to do the same on an iPhone and it is likely partially because battery life could be compromised by allowing all apps to run in the background.
If you want to throw battery caution to the wind and make your Apple smartphone a lot more productive, you need a jailbroken iPhone and a visit to Cydia (an application on the jailbroken device) where you can download this free app just by searching for it.
In a test, I set the application to switch between apps whenever I shake the phone and this will be very useful in situations where I want to check email and listen to streaming music such as Pandora at the same time. On jailbroken phones you can do this now but if you are in the middle of an email and want to skip a song you have to press the home button and then search for the music app. Now you can just shake to go back and forth.
Another example is the Facebook app on the iPhone - if you want to see a link in Safari - it can be a challenge to get back to the place you left off in Facebook. In a test just now I was able to shake from Safari and go immediately back to Facebook.
This is a huge productivity booster and a massive eliminator of frustration for iPhone users.
Apps like this show how arbitrary restrictions by Apple on which apps to allow to run in the background just cause frustration for their customers.
But to be fair to Apple, this new app makes a great phone even greater and yes, it could be the final straw which makes jailbreaking totally worth it.As you can see, there are many ways to use this application... It is highly configurable in fact.