Force.com Labs recently released the Project Milestones application. If your looking for a lightweight project management tool for your SFDC org, it's pretty handy. You can create Projects, milestones, tasks, expenses, and time entries. It has support to create XML exports/Imports of your projects, as well as task creation via email services out of the box. It also leverages Chatter and can auto-follow tasks for assigned users which is neat.
I used a preview / alpha build on a small Project recently and it worked well. It's not meant to replace something like MS Project or Primevera Teamplay. But for a free lightweight app its a nice pickup.