I've been using this a while but it was only when a friend said how good it looked did I think of tidying up the code and releasing it as a plugin.
The script simply displays the 'monthly archive' in a different manner (hence the name!) - there is a table for each year and then a cell for each month in that year that has posts. That's a poor explanation - head over to my site to see what it does and to download it.
I based it on an old MT script, so 95% of the thanks have to go to the original author. Hell, it might even have been pluginarised already, but I couldn't find it.
If you have any problems then leave a comment on the page I linked to above.