pathartl

Accordion Archive Widget is exactly what the name implies. It recreates the stock archive widget, but this time around the widget is laid out as an accordion. It gives you the years of the archive, and then when the year is clicked it expands to show the months.
Development can be tracked at http://github.com/pathartl/accordion-archive-widget

Accordion Archive Widget

An archive widget that reconstructs the stock widget into an accordion by year with month children.

littlejon

LJ Multi Column Archive is a WordPress plugin/widget that allows you to display your archive list with multiple columns.
I developed this plugin as I wanted to make use of the space I had in the sidebar more effectively, while still keeping the links to all the archives available.
Depending on how old you blog is you can set the Archives to display either a list of Months with Posts, a list of Years with Posts, or a list of Days with posts.
Archive links can be shown with or without post counts.

LJ Multi Column Archive

LJ Multi Column Archive is a Wordpress plugin/widget that allows you to display your archive list with multiple columns.

Accordion Archive Widget vs LJ Multi Column Archive