I don’t expect this to be something a lot of people will be interested in, but I did it for a project and I’m posting it in case anyone is.
It’s jQuery JS, EE template code / HTML, CSS, and an EE plugin for expanding / collapsing archive navigation. Click here for a demo.
The way this is written requires jQuery 1.4+ [for jQuery.proxy()], but it can pretty easily be modified to work with an older version.
The plugin just provides a count of the entries for each month. I couldn’t see a built in way (an efficient way at least) to do it.