I have heard good reports abou this open source software
http://www.activecollab.com/

activeCollab

Collab is an easy to use, web based, open source collaboration and project management tool. Set up an environment where you, your team and your clients can collaborate on active projects using a set of simple, functional tools. 100% free!

From the Net: Take a look at the activeCollab writeup and screencast done by Eli Foner to learn more about activeCollab and see activeCollab alpha1 in action.

Highlights
Easy to install and easy to use
Web-based. After installation only thing you'll need to use it is web browser
100% open source, 100% free
No limitations on number of projects, clients, team members...
Per project permissions


Glenn