Beside the modules that come with DNN, I don't seem to be able to find good quality free open source modules. The ones I find are either of poor quality, lack any interesting features or so old they don't work with the latest DNN versions.

Other CMS's like Joomla and Drupal have lots of free good  modules and have active volunteer developers. Why is DNN lacking in the same area?

Have you looked at codeplex? There are modules good quality, intresting features working with latest DNN.

What are you looking after?

Probably because you're thinking Drupal and Joomla "have lots of free good  modules."  They don't really.  There are quite a few very similar modules, but why would you need more than one forum module?  And there are quite a few modules for Drupal/Joomla that already exist in the DNN core.  But there really aren't many decent quality modules with active volunteers when you actually look at them.

The other issue is you are making "free" a requirement.  The non-LAMP world has very few "free" commodities.  And really, they aren't as expected in the Windows world.  On the other hand, there aren't a dozen forks of DNN, there aren't a billion copycat themes/skins and the good modules that are developed are rarely abandoned.

Somebody once told me that the "free" world is composed of amateurs and idealists, while the commerical world is composed of professionals and realists.  It's not that sharply defined, but that's not a bad definition.

The others have pointed to many resources for not just good, but great modules that are free.  You'll find some notable gaps, but there's a lot out there, just not well organized.

Jeff