Windows only: Free application iWatchSyncer monitors any folder on your computer and automatically adds new MP3s to your iTunes library, bridging a very annoying gap in the default iTunes behaviour.
Unlike most modern media player applications that sport the elementary ability to set up a folder (or folders) to watch for new content, you've got to manually add any new MP3s to iTunes unless you purchase said MP3 from the iTunes Store. (Hmm, I wonder why that might be.) iWatchSyncer monitors any folder on your computer and automatically adds any new track it sees there—meaning that whether you downloaded some new tracks of the internet or bought a new album via some source other than iTunes, you don't have to hassle with the manual import every time.
Previously mentioned iTunes Folder Watch offers the same functionality (and then some), but it's always good to have options. iWatchSyncer is a free download, Windows only, requires .NET 3.5.