Link Harvest will go through all of your posts and pages and compile a list of all external links. It will then create a live updating linkroll for you, based on your actual linking activity. The more you link to a particular site, the higher it will appear on the list, which can be shown in a full page display or within your sidebar. The plugin also includes tool to view the individual links to each site and the pages on which the links appear.

This is a complicated plugin, make sure you read the README. You can see it in action here.