WdCalendar-rails is a project mainly written in ..., it's free.
Rails 3.1 wdCalendar
IMPORTANT UPDATE: After working on this for a while, I discovered fullcalendar_rails. FullCalendar is much easier to integrate. I am discontinuing this project. I strongly recommend checking out fullcalendar_rails at https://github.com/vinsol/fullcalendar_rails