It isn't free or even really cheap, but I've been using and selling Goldmine for a lot of years.
[url:http://www.frontrange.com/goldmine/ look for Goldmine Business Contact manager (not the corporate edition). They have become somewhat MS-centric, but everything still installs and runs fine on and from an SME server. Not only multiple calendaring, but a search function to see the next time x, y, and z are free for 2 hours in the morning kind of thing. Much more capable and sophisticated than outlook/exchange - write me offline if you have questions.
Regards,
Tom