On drupal.org we have much less spammers than other websites. One reason is the fact that we do not allow anonymous users to post anything and that every user needs a valid mail address in order to use his account.
This poses the question: Which email providers to our spammers use?
Luckily, this is rather easy to answer:
mysql> select substring_index(substring_index(init, '@', -1), '.', 1) as provider, count(substring_index(substring_index(init, '@', -1), '.', 1)) as count from users where status = 0 and login != 0 group by provider order by count;