Spark! is a multi-user, multi-group calendaring environment. Originally designed for Washington University in St. Louis, The first version of Spark! currently serves over 5000 students and has reached some serious limitations in design.
This new version of Spark! is being rewritten from the ground up to fix many of the original problems and bugs of the first version.
It's our hope that others will join this project and help create a robust calendaring system for use in any large-user environment.
A tenative roadmap can be seen here. As you can see, I expect development to move very quickly at first so that a good foundation is established and then built upon over time.