FYI, plancalendar is cgi based calendar, and monekt calendar is little bit unstable from my opinion I tried to use the features, but I couldn’t make it work, and I’m using the latest FireFox browser.
Your best bet will be
http://www.k5n.us/webcalendar.php or maybe
http://phpicalendar.net/
If you want to integrate it, you need to learn the core programming on tha third party calendar first, so that you know how to hijackthir user base to use the on povided by MODx, or you can set them up separate with separate user database as well. So far there has been no integration between those third party calendar with MODx yet. If you want, you can use the one that we built in here, which is located in this forum thread, but it doesn’t has all the ability to make recuring events and etc like all 3rd party system available. So in the end it’s al depend on your needs. My suggestion will be, use the right tool for the right job