I'm testing using Disqus for commenting here. Drupal's core comment module is adequate but Disqus provides a number of features, particluarly integration with Facebook and Twitter, that would require more work otherwise. There's even a Disqus module that integrates the plugin as a block. While it doesn't integrate with Mollom, it does integrate with Akismet, but the latter is only free if you do not display advertisements. I'm curious to see if it has ANY spam blocking built in.