This is a cool tool. It is basically a notice board with cards stuck on it. And the whole thing is on the web. Just using the web browser. Really cool stuff.

https://trello.com/

Here is another tool.
http://openatrium.com/

Simple and easy to use.
http://www.projectpier.org/
http://www.clockingit.com/

Quite comprehensive.
http://www.fengoffice.com/
http://www.codendi.com

Here some more, need to look into them.
http://www.egroupware.org/community_edition/
http://trac.edgewall.org/
http://projecthq.org/
http://www.kforgeproject.com/what-is-kforge/
http://www.redmine.org/
http://collabtive.o-dyn.de/about.php