PowerCard for Mac

From Random Ideas:

PowerCard is a new revolution in Project Management Software. Instead of managing your project through trees and lists of data, you use an innovative and intuitive "task card" based interface, where you write cards that represent the tasks you need to accomplish and slide them around, as if on a conference table, to organize and prioritize them.

This method is commonly used in Extreme Programming, but can be used for any project.

Additionally, PowerCard is the first ever "Rendezvous Enabled" project management application. Allowing multiple users to edit the same project together simultaneously with no configuration.