iRealb is probably my favorite on ipad, i use it all the time. You can download tons of tunes from their forum . I also usually transcribe and write harmonies straight to this app. Makes it very handy if say a vocalist wants to play in other key because you can transpose the charts.
Also iGigBook is cool for sheet music since it has an index of most realbooks. You can type the name of the tune and there you have it opened. But you have to import your own PDF realbooks.
Meteor is a great multitrack app. As cool as the wizbang features of Garage Band are, I like Meteor better.
IRealBook is an awesome app, the one that made me buy an Ipod Touch
+1 on Amazing Slow Downer, Animoog, IStrobosoft, SampleWiz, Nano Studio.
VoiceJam is a great looping/jamming tool. Everyday looper is cool, with one of the great video demos ever
Mobile Mouse. Ipad as mouse and control surface.
Beatmaker2 is beat making/idea tool
ISwitter is a web browser that allows you to play flash...great app. Only works via wifi, so data package won't help you there if you got a 3g Ipad.
TuneIn Radio is radio streaming app that allows you to timer record radio from all over the world. Sometimes I use it for local stuff that doesn't have strong transmitter, too.
Evernote is a note taking app that syncs with PC/Mac. You can use audio notes and photos, too.