Akismet is a spam filter originally intended for blog comment & trackback spam (for WordPress), but now is also used for forum and link submission spam and other things. I'm in the process of writing an implementation that will be used for sites that don't require users to register to add links or reviews or have auto-validation turned on.

When a link or review is submitted, the form data (along with environment variables) is sent to Akismet and analyzed. The return data is a true/false string.

If it's been flagged as spam, the plugin will take one of two actions depending on your config setting:

-reject the submission automatically
-accept the submission, but require validation (helpful for building up accuracy)

If status couldn't be checked (due to server error, etc.), the record will be added but require validation.

Otherwise, the record is auto-validated as normal.

You have the option to train Akismet by reporting missed spam, or records incorrectly marked as spam ("ham").

I expect to have a completed plugin ready by the weekend, but I haven't yet decided on a price (if any).

In the meantime, have a look at:

http://akismet.com/

Philip
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Limecat is not pleased.