This is a small and flexible calendar script that can be. It is so flexible and small that you can fit it in any design and page size.
It allows running as popup, or being built in your page or even in a form in your script. In the new version you can also set hour and minte of the event.
MicroCal is small, very flexible and fully customisable.
Also hosted as a Google Code project.
Requirements: PHP 4 or above