I'll mention one that probably no one else will: Email Verify. It checks to see if they enter a valid and reachable email address. It has no visible component, so it may not even be on your radar, but I consider it indispensable in the war against spam.

Nancy

Injustice anywhere is a threat to justice everywhere. -- Dr. Martin L. King, Jr.

From: Jim Ruby <jim@v-community.com>
To: support@drupal.org
Sent: Tuesday, September 20, 2011 5:02 PM
Subject: [support] attention any blind users on the list or site administrators spam prevention

Hi, I have seen a few posters on here with visual impairments. I would like
to use some sort of spam prevention on my Drupal 7 site and would like to
know what has worked best for you? I'm currently sifting through the spam
prevention modules now trying to decide on the one I will be using for the
contact form.

Thank you for your time and suggestions.

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