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.
Creating category and Multiple inboxes
Go to settings/labels to create different labels for each category, and use unique colors to differenciate each of the category.
From setting/labs you can enable Multiple Inboxes so that you can see all those categories next to main panel.
Marking and Prioritising
We can use starts to mark each of the email as per their status, e.g. for incomplete task I usually mark exclamation, and completed as tick. I use blue star for less importamt and red star for highly important. We can still use labels to as per the priority level.
Creating Task and full-screen view
We can create task simply by clicking More/add to the task. Task always appears on the right hand side of inbox. If you want to see whole task in a new tab, you can view that here.
There is option on task to create new list and check related mail.
Not Satisfied with these?
Find a list of 50 different third party apps to make your gmail inbox more productive here