Basecamp

Basecamp is a great web-based project management system that was developed by 37signals. It's great to stay organized and in communication with everyone that you're working with and Basecamp does it very well. You can get started with a free account that will work with one project and then move up into the more capable accounts to manage multiple projects. I've been using it with a project that I'm working on now and it's helping me quite a bit.