We all link to external sites from our own posts. Not all your visitors will click on your links though. The ThumbSniper plugin is a cool script that adds link previews to your site, so your visitors will be able to decide whether they should click on your links. It uses jQuery to make your site more dynamic.
The ThumbSniper works automatically. It adds preview screenshots of your links as tooltips. Whether you should use it on every site is another story though.