Here's a useful new addition to the wealth of iPhone homebrew apps out there. iPhone Backgrounder is a code application that lets you run multiple iPhone apps at once. That means even if you exit, say, Pandora, you'll still be able to listen to it while you're running another application.
This is a code app, so there's no icon for it. To use iPhone Backgrounder, open up the app you want to use and hold down the Home button until you get a pop-up saying "Backgrounding Enabled (Continue holding to force quit). If you want to disable it, just hold down the Home button again.
One thing: Backgrounder will be disabled if you respring your SpringBoard. Also, there's a chance that an application would immediately quit after you hold down the Home button. According to developer gaizin, this may be caused by Intelliscreen's App Launch Boost feature, which can be disabled in Intelliscreen's Performance page.
The catch here is that running multiple apps, even in background mode, can drain your iPhone's battery faster than usual. It'll also make you device run slower. If that doesn't bother you one bit, check this app out. It's available on Cydia for free.