bgribaudo

Attempts to prevent automated SPAM by requiring visitors to answer a question of your choosing in order for their comment to be accepted. Presumably, spambots will be unable to correctly answer this question and so comments they attempt to post will be rejected.
Only regular comments from non-logged-in users are filtered. Pingbacks, trackbacks and authenticated users will not be prompted to answer the verification question.
Privacy Note: This plugin does not call home or communicate with any external servers or services. All processing is done locally within your WordPress site.

Super Simple SPAM Stopper

Prevents automated SPAM by requiring visitors to answer a question of your choosing (plaintext or regex).

MikesPickz

MP Spam Be Gone is the simplest, most effective Spam blocker without difficult CAPTCHA responses needed by the user. This plugin was inspired by some of the best Anti-Spam plugins available on WordPress.
Inspirations: 
Spam Free WordPress by Todd Lahman, LLC 
Growmap Anti Spambot Plugin by Andy Bailey 
Simple Trackback Validation with Topsy Blocker by Tobias Koelligan
This plugin has aspects from all of the above mentioned plugins to form the most complete and dead simple Anti Spam plugin on WordPress. This plugin doesn’t write anything to your database while preventing comment spam and trackback spam.
From Growmap Anti Spambot Plugin, I used the concept of having a checkbox display to the user in order to prevent automated Spam. To me this is the best way (and least intrusive) to completely prevent automated spam from ever even being written to the database in the spam que. It is physically impossible to get a false positive, since you either checked the button or not. The flaw with this plugin is that it does not work with Javascript disabled.
To combat that flaw, I use the copy and paste password fields from the Spam Free WordPress plugin. These fields are in a <code><noscript></code> tag, and only appear if Javascript is disabled. Unlike, the Spam Free WordPress plugin, these passwords are generated on the fly and never stored in the database.
These plugins primarily stop comment spam. In order to stop trackback spam, I added the IP address check, URL check, and Topsy check functions from Simple Trackback Validation with Topsy Blocker. If a trackback is identified as spam because of the IP address or Topsy, it is deleted and never written to the database. If a trackback is identified as spam because of the URL, it is marked as spam and sent to the Spam que. The reason that these are not automatically deleted, is because it is possible that the referring page is using a shortlink and therefore the full URL would not appear on the page in the first place.
When combined, this plugin will stop all automated spam and trackbacks.
It is intended to replace the above plugins.
You can’t find a plugin simpler than this – set it and forget it. No options = no modifications to the database.

MP Spam Be Gone

MP Spam Be Gone is the simplest, most effective Spam blocker.

Super Simple SPAM Stopper vs MP Spam Be Gone