The best I've found are:
i4e - A character sheet application that's very customizable and very nice. Imports from the CB file and allows full editing once it's been imported. Runs locally, so for an iTouch, it beats out iplay4e for not requiring wifi. $5 cost, which is pretty reasonable, especially since they provide solid updates and have gone above what other character sheets do.
Compendium - From the same guys who did i4e comes the compendium. This one is free, iirc, or only $1. It requires your ddi password and connects to the compendium for easy browsing through your mobile device. Does require wifi, so no using it when not on a network if you're using the iTouch.
There are various dicerollers, but most are meh-to-ok. I prefer to roll the real ones if I can. Pick whichever interests you the most. I use TRPG Dices....yes, they actually named it dices....sooo lame.
Other things of use:
OneNote for iPod/iPhone is great for campaign tracking, as it stores in the cloud version on windows live. Seems to function slightly better than using google-docs (they really need an app for the googledocs/calendar. The webpage works, but isn't as ideal)
Also, a twitter account with various D&D bloggers and designers is awesome. searching the #dnd hashtag is always great for inspiration.