If there is one app that “completes” the Ipod Touch, its Stanza. You already have music, videos and photos, but with this you can also read books. Excellent.
Saisuke is better than the calendar that comes with the iTouch, you can use this to keep all your plans. To Do’s– very useful for writing things you need to, err, do.
iAnki- reviewing flashcards on the move. Great for learning anything. I also got iKanji Touch recently, and there is Kotoba!, a free Japanese dictionary, to complete your study needs.
For sports I’ve got iFooty, and a web link to Yahoo.co.jp’s Pro Yakyu page, that both deliver live scores and stats when games are on.
Although Stanza is good for most things, it can’t read non-English PDFs- I use Air Sharing for this.