I've had my iPad just under a month, with an eye towards using it at my game table. So far , the apps that have been useful to me are:
dicenomicon (absolutely worth the cost)
PDF reader (iBooks allows you to import PDFs, but sometimes they don't come up correctly, haven't had a problem wit PDF reader).
dungeon mapp (you can draw as you go, and set plastic minis on your iPad, if you wish - though I wouldn't advise metal minis)
inotes - A spruced up version of the basic notebook app that comes with the iPad. Notable as you can highlight text, draw/insert pictures and keep separate notebooks for different campaigns, adventures or whatnot.
numbers is an app you can purchase that does spreadsheets, useful for building monsters, character sheets or any other data tracking you may need to do.
of course, the built-in iTunes will allow you to play background music or other sounds for your game. If radio rivendell has an app (or something like Pandora radio), you don't even have to download music to you iPad.
Some of the apps I tried, but didn't like:
DM Toolkit - cumbersome, unintuitive and unless you play 4e, you have to hand-enter all info. You could do better with the built-in notebook app
Cartographer - you can make some great looking maps and it has "mini" images you can drag around for PCs and monsters, but it would be very difficult to replicate a published adventure map with the templates it comes with, and there is no free version.