I'd like to take the opportunity to thank Mollom for their excellent free service helping me to prevent spam comments. As you can see in the following diagram spammers have become huge fans of workaround.org. So I had to enable the moderation of comments, too. Up to 300 spam comments per day are prevented by Mollom so I just have to deal with 1-2 spam comments manually.
A hint for Drupal 7 administrators: the "Homepage" field in comments is pretty useless and just an invitation for spammers. Fortunately in Drupal 7 you can disable the field pretty easily. Just edit the respective content type and set "Anonymous commenting" to "Anonymous posters may not enter their contact information". You will also want to enable a content filter removing all A-HREF links from the body.
If anyone knows further good anti-spam plugins for Drupal - drop me a comment. I'd even be happy if I could just drop comments that contain "http".