If you are a jazz musician with an iPad, you definitely want the iRealbook app. Saves me from having to carry a bunch of books to gigs. The accompaniment feature has been very useful as well.
Man how many times has iRealbook saved our backsides lol I've also got a pdf of the 6th Edition real book in ibooks which is handy too. For charts I more use that and use irealbook for practice actually. Buying that playalong pack is an awesome practice tool!
Also of course garageband. For $5 it's insane what you can get.
I also find that personally I struggle to use 1.5gb of data a month so the streams are great too! I use jazz.fm91 and FLRP
Also Karajan is the best ear training app out...And one last one is called Tritone. Rather than aural training it asks you endless interval questions. Invaluable start to the morning, get your key signiture brains working