For 4e, the iPad has a few key apps IMO:
1- The Compendium app. The search is actually better than the online search in the actual compendium, IMO.
2. i4e for character sheets.
3. A good PDF reader or two. I'm using iAnnotate PDF and Goodreader.
4. Evernote for all kinds of notes. Use it in combination with Skitch, which is an image grabber tool very handy.
5. Feedly, or some other pretty RSS reader.
6. A Twitter app. There's often good stuff going on out there.
Actually, only the first two are 4e related.
I'm a fanboy of Razeware's Battle Map -- it's terrific for quick maps if you're using some sort of digital tool to display maps for games.
There are a handful of random generator apps that are handy to have when you need a quick NPC name and you're not prepared.
Dicenomicon is very, very cool, but I mostly still roll dice. Call me a grognard. We also do initiative in a very lo-tech way, not on the ipad.
Gamer's abacus is pretty cool for tracking a bunch of different information.
I've played around with Comic Life and Keynote, both, to create more visually interesting recaps of previous sessions for my games, but don't often have the time to put in the work.
-rg
PS -- these old columns of mine might have a few more ideas:
http://www.enworld.org/forum/content.php?477-Gamehackery-Convention-Gear
http://www.enworld.org/forum/conten...agazines-Apps-and-Getting-Your-Gaming-Content