The archives WordPress plugin. This plugin creates real posts archives and greatly reduces the DB load since old posts are called only when needed. This plugin is great for sites with thousands of posts!!!
You can read more about it here: http://jeez.eu/2011/03/14/qoolarchives-an-archives-wordpress-plugin.html and you can also visit http://www.qoolsoft.gr for more info
This plugin is available for download to be used on your WordPress self-hosted installation.