I tried looking for just such a tool a while ago without success (but also without much effort).
A quick googling for 'free project manager software' got several interesting links, but I can't do any recommending.

Perhaps this comparison at wikipedia would be a start-
http://en.wikipedia.org/wiki/List_of_project_management_software
Just having a list of names is a good thing to start with.

Sorry I can't help more than that- I'm sure you've already googled it yourself.

edit:
This guy seems to feel strongly about a couple freeware options.