Basecamp

Made by the same company that created Backpack, Basecamp is a solid project management system. The free version allows you to track one project at a time, but allows an unlimited number of clients.

It's primary functions are to share, prioritize and assign tasks and to schedule due dates and milestones. You can, of course, also share notes within your organization.

If you outgrow the free version, there are several different level of premium packages you can license.

http://www.basecamphq.com/