Ok I see this system on a *lot* of sites; (Example is on iDownloadBlog) What is that system called? and is it wordpress compatible?
The system is known as Disqus. You can download it at http://disqus.com/. It's compatible with Wordpress but you may have to modify the PHP files to get it working.
its disqus, as jamjar said. there is a wordpress plugin http://wordpress.org/plugins/disqus-comment-system/ so you dont have to modify anything. it will also import your existing wordpress comments so that you dont loose them they have a javascript snippet so that you can add disqus to your custom site too
Oh I feel stupid now, I thought Disqus was just a type of social media/blogging like digg etc, I really really like the system/integration.