Description
This is a relatively simple plugin that uses Javascript to make the Archive links in the sidebar collapsable by year and/or month. There are a number of options available, such as having the ability to link to the archive pages or not, and whether or not you want to display individual posts or not. It also supports custom post-types. There are several default styling options built-in which work well with most themes. You can also define your own custom styles.
Demo
I use this plugin in my blog at http://blog.robfelty.com