Finally! An open source, BaseCampHQ.com clone with great promise:
activeCollab 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!
Read more about this PHP based solution here: activeCollab
Only downside is that it requires PHP 5, which is still not available for many server installations.