For me the most Useful apps are
Photos
Camera+
Utils
Orderly - To create your ToDo lists using simple iPhone gestures
System Monitor with Battery Max - To monitor the system activities like memory, cpu, disk, processes
Magnifier Flash - A zoomer
Finance
Easy spending Expense Tracker - for tracking the income/expenses
Music
Make my music - custom music app to auto schedule your playlists
LikeMyMusic - Socialized music player
Travel
Trip Photos - Tag, Search, N Share - A compact app to travel with
Games
Angry Birds
Ninja jump
Cat physics
Fruit Ninja
Cat physics