Sunbird is only available as the Lightning plugin for Thunderbird. It's pretty good now-a-days; I'm using it as my primary CALSCH client. I'm using the Lightning internal "Home" calendar until I find a good CALSCH server I can !selfhost http://bob.jonkman.ca/blogs/2012/10/12/calendaring-server-software/ !googlefree (where's that group when you need it?)