Collapsing Archives
The collapsing Archives plugin creates a hierarchical list of archives organized by year, month, and post. The years and months can be expanded and collapsed dynamically, as you can see in the sidebar on this page. It includes a number of different options, including whether or not to display months and posts.
The plugin is hosted on the official wordpress repository
Bugs, Features?
Please discuss bugs and features on the forum
I have decided to move discussion of my plugins to bbpress, which should be more flexible. Please visit the forum for collapsing archives