A key feature of the iPhone/iPod Touch is the ability to draw Chinese and Japanese characters on its screen. I don't know how useful this is for creating text, but it is an invaluable replacement for radical/stroke analysis when looking up these characters in a dictionary. For Japanese I am using the app Kotoba and for Chinese DianHua.
Other language apps I've found so far that seem especially useful or high quality are the mobile version of Google Translate, the Larousse Dictionnaire de Français, the Classic Greek dictionary Lexiphanes, and the Latin dictionary Lexidium.
Additional suggestions by readers are most welcome.