Concourse is a tool to make planning gatherings easy, even if you don't know the schedule of everyone who can attend, or even who can attend in the first place. It's based on an (application written for Railsday 2006)[http://technomancy.us/47].
To see the original in action, see http://concourse.technomancy.us
OK, so you've got this meeting, right? A bunch of people have to come to this meeting, but you don't know when they are going to be available. So what you do is sign on to Concourse and create a new meeting. You give provide some details and a list of folks you'd like to attend. When you're done, each person gets an email asking them to please enter their availability for the meeting.
Each person can then log in with the URL provided in the email to specify the times they can make it to the meeting. In theory, when everyone had entered their times, the organizer would then be asked to decide upon a time, and his decision would be informed by what everyone had said.
Eval the file concourse.el from within Emacs to get a server running on port 8888 via SLIME. All dependencies are bundled except the JVM (1.5+), Emacs, and SLIME.