Here are three great extensions to help you spice up your Joomla powered blog:
Disqus comment system - allows visitors to add their thoughts about your articles. Start by registering for an account at Disqus and then put those account details into the extension.
Lean more about Disqus comment system
FJ Related Articles Plus - the Joomla core includes a related articles module that displays links on an article page to other similar articles on your site. This extensions takes this further with more features and greater flexibility.
Learn more about FJ Related Articles Plus
Ninja RSS - Syndicator - once again, RSS is built into Joomla as standard but it can be difficult to include your desired articles into one feed. This extension solves this problem by allowing you to select your desired sections/categories and build your own feed.
Learn more about Ninja RSS - Syndicator