Here is a first list with some of my favorite apps

Evernote : app to manage notes, possibility to organize notes in different notebooks, save search queries for reuse, ...
+ simple to use
+ todo checkbox to more easily find todo's
- no list of todos from different notes, but with convention, you can do this with search queries
- pc application necessary to manage different notebooks

Adobe Reader : app to read pdf's
+ possibility to make annotations, highlight with marker, draw on the pdf,...
+ annotated pdf can be sent via mail

Hello ! : app from belgacom to follow up 3G spending
+ good to see what's left from your monthly 2 GB

Flipboard : app that assembles the latest news from the web, and shows it like a magazine
+ very easy to setup
+ can also integrate news from your twitter account

Railtime : app from NMBS for train information

BBSEKE