Mog music service - 320Kbps over wifi with iPad specific app + airplay
Zinio - Magazines
Splashtop - Remote desktop - audio can be setup to come through iPad speakers
Kindle
Slingplayer - see Directv anywhere (it requires a Slingbox)
Pulse - great news/blog reader
Sirius XM - love the favorites page that shows what is playing on other channels
Rowmote Pro - great for controlling Mac mini while on the couch
Eyetv - similar to Slingbox, but has ability to record to mac from Directv
Nighstand - use this for my clock each night
Aquarium
Audiotap - streams ALAC iTunes library in high bitrates (they need an iPad specific app, though.)
All of the Apple apps - Remote, Safari, iPhoto, etc..
Denon app -for controlling receiver without remote