Ask HN: Open Source Pivotal Tracker/Basecamp alternative?

I've had success using Pivotal Tracker, JIRA, and other project management tools in the past. Now I want to install something like that on my own server. I'd like to do it on my shared hosting account, so a PHP solution is preferred, but I would also look into things I can put on my dedicated box. I prefer Pivotal Tracker's interface over the other alternative. And yes, I've also had previous success with Google Docs, but that's not what I'm looking for here - I'm looking for something specifically similar to Pivotal Tracker.