I've suggested you Wunderlist just a few days ago.
iProcrastinate can be an excellent alternative for mac - iPhone users.
In my humble point of view:
- Free mac desktop app.
- It's student-oriented: subjects - tasks - steps
- You can link files (it copy them in a separate folder, but it remembers the location in your computer).
- Calendar view.
- Straightforward sync via dropbox with other devices.
- The developer is on twitter: @craigotis.
- I can't select multiple tasks in order to modify them together.
- Limited in being multiplatform.