A nice looking PHP powered calendar can be found here:
Easy Simple Calendar
Script to buy is $16.95 (so not free, but quite reasonably priced!), and if you want to remove the copyright notice from the popups it's an extra $8.00. Easy to customise and administer. They also offer free installation if you're a complete newbie!
Two free PHP Calendars are listed below:
PHP-Calendar
Jax Calendar
Hope that helps!
Paul