Subscribe to a Language-learning Podcast. iTunes offers any number of podcasts that will help you learn new languages. A quick search on "language" turned up lessons for French, Japanese, Italian, Latin, Tibetan, Turkish, Gaelic and more. On the down side, clearly some of these podcasts are home brew. On the up side, the podcasts are free. You may want to try out several podcast providers before settling on your subscriptions.
Buy a learning audio book. The iTunes store offers a wide range of language courses in their audio book section. These courses are typically of a higher quality than the podcast ones, but they also cost quite a bit more. If your local library lets you, you may want to borrow language audio materials from their collections and use them with your iPod. Ripping policy varies by library, so make sure you know the rules before you rip.
Use OS X-compatible language-learning software. Many top-brand language packages like Berlitz now run on OS X as well as Windows. Interactive software can really help you master certain language skills like grammar via interactive lessons.
Take advantage of OS X's internationalization. OS X is particularly multi-language, multi-alphabet aware. If you're trying to learn new languages, take a few minutes to explore the International system settings pane with it's "input menu" virtual keyboards that will help you type in the language of your choice.
Sign up for a class. The success of iTunes U has made it possible to sign up any number of university and college courses. The use of iPods and podcasts are now so prevalent that Duke now offers an iPod-orientation course for all its language instructors. Unfortunately, some iTunes U courses require that you have an actual student ID for sign-up. (Stanford is one such university.) Some do not. Also unfortunately, you need to perform quite a lot of googling to find public iTunes U classes for any particular language, so be persistent. iTunes does not yet offer a specific iTunes U search engine.