eXtCal is an event/calendar module developed exclusively for XOOPS, it is thus integrated perfectly there. It integrates a fine management of the right user allowing thus a great flexibility in are use. You can view it in live and find support on my web site : Zoullou.net.
This is the third and the last (i hope ) Release Candidate version, don't use on production site. Please report any bug you find by the Bugs Tracker and give your XOOPS and PHP version.