You don't have to buy one, but CleverToDo has the most clever (pun intended) Live Tile available and it is a great tasks app, with recurring tasks, backups, reminders, etc. Show next three tasks on front and inbox, today, next, someday counts on back. Works like a charm. Took less than a minute to move my tasks from my Surround to the L900.
I love OneNote. Been using it for years. But it is not a tasks app. List, maybe. But not to-dos/tasks. Btw, Smart Tile is a good lists app with a Live Tile.
Sent from my Lumia 900 using Board Express