A: this article gives you a list to choose from: http://www.smashingapps.com/2010/02/15/10-free-tools-for-effective-project-management.html
The additional links are in the comments below:
http://www.oneplacehome.com/
https://launch.unifiedinbox.com/
http://openatrium.com/
http://www.gtdagenda.com/
http://www.teamworkpm.net/
etc.
