You must look up Asana...completely free, user friendly, and you can create different work stations under one account which has become very useful with my clients. The can add tasks for me, set due dates, and communicate progress online. There is even a free iPhone app...which made me very happy!
I have looked at zoho, teamwork, sales force and all the other suggestions on here. My client actually introduced me to this and boy am I glad! I won't look back!