iPod.iTunes offers 8 different ways of synchronizing your iTunes library and playlists between both devices, giving you powerful control over what files are moved where and how. All formats are supported, including MP3, AAC, protected AAC (iTMS purchases), Audible books, and video files. PC-formatted iPods are compatible, and all metadata such as your song ratings, play count and album art are preserved in the transfer. This is a great utility for anyone in need of an easy method to clone an iPod, keep iTunes libraries on different Macs in sync or simply back up your iPod's library if it's the only place you keep all your media (Apple sadly doesn't provide a way of doing this).
Check out the full list of features of everything iPod.iTunes is capable of. Another handy trick this app performs is that it can live and run directly from your iPod, in which case your registration ($35 USD) travels with you. A demo is available from crispSofties.