What I use most are web browsers, I have 11 installed but three I use most.
For file management, FileBrowser and DocsToGo.
To control my PC, SplashTop Remote.
Magazines on Zinio.
Books on iBooks, Kindle, Kobo and Stanza.
Drawing with SketchBook, ArtStudio and iPocketDraw.
Slingbox to control a tv and listen on the iPad
Various radio stations that have their own apps.
Quite a few apps for different tasks such as the AppleRemote to control my AppleTV.
I'm really looking forward to GarageBand next...