I stumbled upon this a while ago, and love it's use.

iTunes Library Updater

all it does is scan your iTunes Music Library.xml file and add any new music that you may have put into your music folder. It's pretty sweet if your like me and always downloading or ripping new stuff to your collection, and it runs pretty quick, depending on your library size. I still wish itunes autoupdated itself, but until then, this is a quick and cheap manual way to do it.

A side note, if you remove songs from itunes, but don't delete them from the music folder itself, the next time you run itlu, it'll re-add those songs.