There are plenty of project management & online colaboration tools out there. Wide popular are Basecamp, Assembla, Activecollab etc. But they are mainly designed to suit the requrement of larger agencies.
I’m a google apps user, and I use gmail.com to login my @kriszha mail. I believe most of the freelancer uses gmail either with @gmail.com or google apps like me. Here you go step by step how to productify your gmail inbox to manage your projects, setting up priority, adding to task, and segregate them in category.