A Few Application Ideas

Accounting App:
Full accounting application, with all of the normal accounts, and double entry

HR App:
Managing employees, paid time off, wage, warnings, etc...

Simple Finance App:
Used for everyday use to manage how much is in debit, credit, etc...

Car Database:
Could go into detail such as how to fix certain parts of each car via related tabs, and other information (cost, weight, etc...)

Video Game Database (like WoW database, or a compilation of games):
Record of all games for all systems, or could be game specific (all items in-game, quests, etc...)

Music Database:
Could hold band/song information, link songs to bands and albums, with as much info as possible

Novel Creation App (Thanks Ryan):
Character object, which has sections such as Brief History, Characteristics
Related tabs for friends, foes, events which they're involved in, etc...

Tennis Academy:
Application for tennis schools, could also be used to organize court availability at a certain time

Any other ideas?
Post them here!