Two nice calendar systems for PHP (neither one is free but not too bad of a price):

Calendar Express 2 – Robust system with a full administration panel and multiple categories and different display options (weekly, monthly, yearly events view).

Easy PHP Calendar – Simpler than Calendar Express in its visual appearance and administration. I really like the way this system works. With some hacking around you can get it to display any number of months on a page and change colors and fonts. One plus, also, is that you can install WYSIWYG text editors (you do it yourself).

Know of any others? It’s tough to find a decent one.