I recently purchased the latest iPod touch update in order to access the Application Store. They have a lot of apps for free, but there are also plenty that must be purchased.
Some of the most expensive apps that I have found are language learning apps, specifically apps that are supposed to help with Japanese. For example, the “Wisdom English-Japanese dictionary” costs $23.99 (it only has a 3 star rating, definstely not worth the.money).
The best bang-for-buck application, however, is the wwwJDIc application. It’s the complete online version of the dictionary, it’s open source, it has a nearly infinate number of entries and examples, and best of all, it’s free!
The only significant drawback is that you have to be connected to the Internet. This won’t be an issue with the iPhone 3G users, but iPod touch users might not appreciate being tied to a wireless hotspot. But if you have wireless at home or easy access somewhere else (like a college campus), it’s a safe bet.