RSVPMe is a project mainly written in PHP, it's free.
RSVP system for MODx Revolution
RSVPMe will (supposedly) help you get people registered for events. It does not attempt to be a full scheduling system, it just lets you create events and have people register.
These are things that are planned to happen at some point. There is no 'offical' schedule at the moment.
Build it and install from _build. If you do not know how... then I'd recommend waiting for it to show up in the MODx Revolution Package Manager. (It is not there now as this is still not really ready to be used!)
RSVPMe is distributed as GPL (as MODx Revolution is). It is based off modExtra by Shaun McCormick.