Disqus is a commenting system that offers features improving commenting blog posts in many ways:

• Threaded discussions: users can reply to other comments
• Email replying: bloggers can reply to comments via email
• Shared profiles: The profile that commenter creates in Disqus is shared across all blogs using Disqus. So users can use the same profile to post comments in any blog that uses Disqus.
• Spam control

Installing Disqus is easy:

• Download the DISQUS Comment System WordPress plugin
• Copy the plugin to WordPress plugins folder
• Go to admin panel and activate the plugin
• From Settings->DISQUS->Advanced set your DISQUS API Key and import your current comments to Disqus

Disqus is currently funded by Y Combinator and Union Square Ventures. There are other competing services such as JS-Kit and IntenseDebate.