activeCollab, an open source web-based project management and collaboration tool, which some are calling a potential Basecamp killer. Without even looking at it, I can tell that the best feature over Basecamp seems to be the ability to install it locally, so you're not storing your project files on another company's server. I'm not sure Basecamp has anything to worry about just yet, but I'm willing to try any OSS project management tool. Based on PHP and MySQL.