Not to long ago I was looking out for a good open source project management software for a friend. He was using Basecamp, but did not like it, cause it was not free software and working for a organization which really pushes for free software, he was in the market looking for alternatives. This is when I stumbled on something called ActiveCollab.
ActiveCollab is written in PHP5 and uses MySQL as a backend, I have not had the time to look under the hood. I have not had any unsuspected issues yet with it. Although it still feels like Basecamp 2005'. There is no AJAX, just a little JavaScript here and there. But the road map seems bright. I have also heard a lot of developers are interested in helping the project out. A while back a friend of mine, told me he and some friends at Stanford were developing a GPLed version of Basecamp written in Python, they did not use DJango but instead wrote their own framework.