I decided to list the most attractive Joomla extensions, which I have used at Kirnu

Already used:

Commenting: yvComment v.1.16.0 (1.5 native)

Needs a little more development. Especially the thing, that comments need own category, makes the article management a little more difficult. Other things are, that there is no need for topic for comment. 

Section & Category management: SectionEx (1.5 native)

Very good tool  for section management. 

Facebook & Joomla: JoomlaFacebook (1.5 native)

Allows users to login with Facebook account.