I haven't bought anything yet, as my Ipad will be months away still, but the things I am certainly going to buy/download:
-Converter for Ipad
-Pages
-Keynote
-Numbers
-Brushes
-some kind of calculator app
-one of the Periodic Table apps
-the BBC app
-Ibooks, when/if it will be available in my country...
-If Ibooks isn't available, I'll just have to do with Stanza
-Chess.com app, if they will make a version for the Ipad, else I'll just use the Iphone version.
-Encyclopedia for Ipad, because I want to be able to quickly look things up, offline.
-Dictionary! for Ipad, however I will also try Dictionary.com, but again I like these things to work offline too, plus it will be faster.
-I will likely buy a version of Risk or a similar strategy game, if I can find one. I'd like solo and multi-player options.
-hopefully I can find a rpg-dice random number generator. If I can't find one for the Ipad, I'll just use my iphone app.