As a hot air balloon pilot, I have all of my FAA FAR/AIM documents, sectional maps, aviation weather/flight-related PDF's, and text articles from back-issues of "Ballooning Life" all stored in GoodReader. The iPad also goes in the "chase crew vehicle" to be used with Google Maps while they chase the hot air balloon.
As a software developer, I have the current tech books I'm working through such as iPhone/iPad app dev books and ASP.NET MVC books. I also have the iTunesU courses for iOS dev that I watch on the iPad while I write code on the MBP.
For news & current events, it's Flipboard, Twitterific, and Pulse.
For games, it's Crosswords, Sudoku, Dungeon Hunter HD, Flight Control, Harbor Master, Ravensword... et al.
As a casual guitar player, Tab Toolkit rocks for guitar tablature, and then miniSynth Pro for creating tunes on the fly for kicks.
For work, I've completely replaced paper notebooks in all my meetings and use Evernote for meeting notes. I've even replaced my Moleskine that I used to journal in and use "Chapters" app now.
When I'm feeling creative, it's Sketchbook Pro and Artboard.
Then there's Netflix, ABC Player and CineXPlayer for the 5 minutes of downtime I get in a day, hehe.
