I've so far successfully stiff-armed the impulse to buy an iPad and though I can't rule that out I do appreciate my iPod. My favorite apps? Glad you asked.
1) iBreviary - it has the Daily Office as well as the Mass readings.
2) Olive Tree Bible Reader - while the Jerusalem translation isn't available, it's pretty fantastic how they integrated the app so that while you're reading a given Bible passage you can see the New English Translation notes, or read the Matthew Henry commentary. I have three translations available: Douay-Rheims (free), the NRSV ($14) and the NET (free).
3) Rate Beer - the name pretty much says it all. Beer Universe is good too.
4) Score Mobile - I can not only check the scores of professional and collegiate games but also the individual statistics of the players involved. Pretty schweet app. Baseball Statistics 2009 contains lifetime statistics of every major league baseball player.
5) Twitterific - very attractive interface for Twitter. Sometimes even makes my tweets look good.
6) Fluent News - a fine news aggregator; Start Up! is also good.
7) Instapaper - allows you to read anything on the web on the iPod/iPhone in a decent font.
8) Audio Books - can listen to public domain books for free! Been lately enjoying Albert Payson Terhune's "His Dog".