I think the Knitting Help app is for both iPhone and iPad.
Let's see, I have Knitting Help, knitCompanion, Raglanify, and iKnit Gauge. It's questionable whether the gauge is useful and I haven't used the other two yet. Someone else recommended them to me so I got them. knitCompanion is supposed to be awesome for charts.
I also use Good Reader (iPad) as my PDF reader. You can connect to your Ravelry library! I haven't completely explored it yet, but all my knitting friends use it as well.
Other than that I have a bunch of games and some other cool apps. What about you?