One perplexing omission from the iPhone OS's feature set is the ability for applications to run in the background.
Oh, sure, we've all heard Steve's excellent reasons: "Shut up, you scum. It's impossible! There's not a single mobile OS that supports applications running in the background. None! It would require magic. I don't care if you want it. I only use one application at a time, and It's called the iPhone [taps an emphasizing ellipses on his own cadaverous chest], not the youPhone."
Jobs' rationale is certainly compelling, but anyone trying to have an instant messaging conversation on the iPhone, or quickly check their email while playing a round of Aurora Feint, has wished Apple had somehow figured out how to include the functionality.
No need to wait around for Cupertino to figure it out, though. A plucky and resourceful Cydia programmer has released software for jailbroken iPhones that allows applications to continue running in the background. After installing the program, you simply keep the home button pressed to toggle the running app into background mode.
It actually works a peach, as long as you don't abuse it. Keeping Twitteriffic open in the background while I played some Tap Tap worked wonderfully, although trying to keep Aurora Feint open while playing Super Monkey Ball required an iPhone reset.
So there's a few problems. But if you can remember to use the iPhone Backgrounder lightly for things like IM and Twitter apps, this is another strong reason why Jailbreaking your iPhone is still totally viable in the age of the App Store.
iPhone Backgrounder [Google Code]