Some free alternatives: 1) Trello (already mentioned) 2) Remember The Milk (free for 2 people) 3) Todoist 4) PivotalTracker

This looks promising, however, I'm not sure about the pricing model, $99 per month (IF billed annually) for 10 designs (10 artboards)?

Yes, one artboard usually correspond to one webpage. How much were you willing to pay? Is this a problem you have every week?