The horde Beta version has about everything you could imagine

, but you need to be willing to run a beta version (it's pretty close to release from what I understand), and you'll probably need to pay some $$ to have the LP people install it since its not a simple upload

. Lets see, it has a calendar, image gallery, news/weather, phpNuke like front-end, comics, a wiki, forums, and a bunch of other stuff most of which is only useful if your a service provider.
But perhaps you would be better off installing a calendar type client that is separate from the email client.