jQuery Sparkle seems to be a good bet at providing a clean elegant solution for what you're asking.
It includes a very nice and slim jQuery Events Calendar based on the jQuery UI DatePicker. So you can highlight days of the jQuery UI DatePicker which have events and assign events to them - such that you could use onmouseover to display information about the events on that day.
As it is built on the jQuery UI DatePicker you could also use it to select days and use all the power and customisation as the DatePicker naturally has! It also supports AJAX or JSON objects to recieve data about the events.
You can find a demo of this functionality right here:
http://www.balupton.com/sandbox/jquery-sparkle/demo/#eventcalendar
(should be right down the bottom last time I checked)
It's open source under the AGPL licence, so you can feel free to grab what you need out of it worry free! :-) It's also actively developed on a day to day basis so you will never be short on support.
But most importantly it is a DRY Plugin/Effect Framework to allow you to develop plugins and extensions much more easily. So hope this helps to achieve that goal!
You can find more about jQuery Sparkle, and see some its demos right here at its homepage:
http://www.balupton.com/projects/jquery-sparkle
Hope I've helped!