The Warning module is a simple module that can add messages (admonitions) to common forms (like the user registration or comment submission form) to provide the user with important information like submission policy, the fact that content is moderated or simply a scare message to get rid of spammers.
I wrote this module because my personal blog was listed in do-follow lists and generated up to 30 spam comments per day that were not filtered by Mollom. After adding a simple (default) warning for comment submission about the fact that I do not tolerate unrelated links and the comments are moderated, this number dropped to 3 spam comments on average.
Comment spam is a real nightmare because the content of the comment may look related, and sometimes are very informative, but the homepage (or links in the content) point to unrelated sites. And if you can reduce the number of comments, you reduce the overhead of moderation.
The Warning module allows to customize the message text and also specify the message type (Caution, Disclaimer, Important, Note, Tip or Warning).