I've just finished shoehorning Quick Calendar into the zenPage sidebar to highlight (and provide links to) dates on which new content has been added.
Quick Calendar is a very simple, ajax-enabled PHP-based calendar, easily theme-able, and can be hooked into a dataset to give a calendar view of the data.
So, here's the deal, this is far from what I'd consider a proper plugin, but it's probably 85 to 90% functional in context of zen and what it's supposed to do/represent. I think for somebody with some zen plugin dev experience, bringing it to the finish line as a proper plugin would not be too difficult. There is a bit of fat to trim out (stuff that will never be used as part of zen), but most of the work is in packaging and variablizing some of the static bits (i.e.: database login, paths, etc).
The way I'm using it, it feels and looks native to zenPage, preserves the month displayed on page refresh where it seems appropriate to do so, and respects permissions based on logged-in user.
If anyone wants to hack this into their own site, I'll assist the best I can. Better yet, if anyone wants to carry this forward, I'd be happy to collaborate (and maybe learn a little along the way).
Thanks,
Adam
www.PigsLipstick.com