Basicly I would say ANY application.
- accounting
- word processing
- spreadsheet
- presentations
- graphics editing
- video editing.
Trick: develop something like Microsoft Office, but running from a web browser. People can save files local or on the server, depending on installation and space. This way they could run Office or Photoshop from a pda.
It's all there... you ust need to be the first to get it to commercial shape.