I've also recently been looking into this. The below link is not mine.
This page gives a nice overview of most, but not all, systems out there. Some do more than just ToDo lists, but still have that functionality. You could also modify/hack a bug-reporting system to work pretty easily.
I wanted to install Trac, but it required too many other pieces of software to be run at DreamHost, so that's out.
I've been trying to get Roundup (Python-based) to install for the past few days, and I'm so close to having it working, but I still get errors and I'm ready to give up.
So I've been taking a closer look at a few more:
WebCollab (available at opensourcecms.com to demo) - webcollab.sourceforge.net
FlySpray - http://flyspray.rocks.cc/?p=Main_Page
Eventum - http://dev.mysql.com/downloads/other/eventum/
Right now I'm probably leaning more towards FlySpray than anything else.