YUI Calendar Component
Before I settled on using the Script.aculo.us method for my drag-n-drop sorting demo I had played with the Yahoo! UI library and their sorting methods.
If you've not already, then it's worth taking a look at the Yahoo! UI library. There's more to it than drag and drop. As well as Ajax, DOM manipulation, event handling and animation there's also additional components, such as tree views and a calendar plugin.
I used the calendar component the other day. It wasn't exactly easy to figure out, but it's powerful and easy to internationalise and customise, which I needed to do.
If you've got the space to add more weight to your page-load then have a look at adding the YUI. You won't be able to keep up with the next phase of the web if you don't have something like this in your toolbox!
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I'm using the calendar component on a fundraiser site I created for an animal rescue group I'm involved in. I'm sure if you really want to take it aparet and understand it, it IS difficult to figure out, but if your skills are more along my level (used to be good with JavaScript but have spent 5+ years on client-only apps) than Jake's, don't let it scare you. It's really easy to just pop in the code from the example files and use as-is. I just imported the js files into a single JavaScript script library and added it as a resource to the form's JSHeader.