The webapp offers all the features you'd probably need from a to-do app, including multiple lists, sharing, and email reminders (the daily agenda is very useful, telling you how many hours you have to work with today and what your tasks are). Ranking tasks is dead-simple: Drag them to the top or bottom of the list to assign their priority.
The Planner is Taskk's killer feature. When you add a task, you're forced to really think about how much time it will take. Then set the number of hours you have to working on your tasks each day and Taskk will magically plan your days for you based on how much time each task takes, which ones have the highest priority, and how much time you have to work with.
Taskk is in invite-only beta now, but the first 2,000 Lifehacker readers can get in the door now with the link below. Give it a try and let us know what you think in the comments.