Ever purchase an app on your iPhone and find out later that it’s magically shown up in iTunes? This is because your iPhone automatically transfers new app purchases during a sync. But what happens if the apps in iTunes are lost? Turns out you can invoke the magic app sync anytime you want. Simply right-click your iPhone in the iTunes Devices section and choose “Transfer Purchases.” Any application on your iPhone that isn’t in iTunes will immediately be moved.
Of course you can re-download any app in iTunes for free, but then you already knew that. ;]
via iphonealley.com