copyThing (was iPod.iTunes) has been designed to make it super-simple to copy content from iPhones, iPads, and iPods to any iTunes library. Its user interface makes it incredibly obvious what's on your device and what's missing in iTunes. For example, to copy all missing content (without duplicates), just press Command-A and click the Transfer button. That's all!
You can, of course, also copy only desired media and/or playlists.