We have used phpCollab for many years and can't image running a project without it.  It is the tool that runs our Client Extranet.

"phpCollab is an open-source internet-enabled collaboration workspace for project teams. Modeled on Macromedia Sitespring, phpCollab architecture allows for the consulting team to share information with each other in one space and publish that information, when desired, to another space for the client. phpCollab encompasses the most important aspects of project management, such as task planning and document sharing, and hooks into other open source applications for bug tracking and content management, for ongoing project support."
- phpCollab

Some of phpCollab's features include:

• A client interface as well as a professional user's interface
• Granular task managment
• Email alerts and the ability to carbon copy others on updates
• Gannt charts
• Calendaring
• Reports - including exporting to PDF
• Ability to copy tasks
• Ability to prioritize tasks
• A clean, easy to use Dashboard
• Ability to add notes and associate them with projects
• Ability to conduct discussions related to projects
• Document managment and peer review of documents.