Using the Google Analytics functionalities (you need to have a Google
Analytics account and the Google Analytics code installed in your blog)
the plugin add a little javascript code the track all the clicks on
links going outside your site.
It can also force the outbound links open in a new window, without change a
byte in your theme or posts.
More here:
http://www.satollo.net/plugins/outbound-links.
Upload the plugin folder into your "wp-content/plugins".
Log in to Wordpress Administration area, choose "Plugins" from the main menu, find "Outbound Links",
and click the "Activate" button.
Go to the options page and activate the funzionalities you prefer.