Akismet is all the rage among the kids these days for blocking comment spam. Started by the founder of Wordpress, Matt Mullenweg, Akismet is a RESTful web service used to filter comment spam. Simply submit a comment to the service and it will give you a thumbs up or thumbs down on whether it thinks the comment is spam.
In order to use Akismet you need to sign up for a free non-commercial API key with WordPress and hope that your blog engine supports the Akismet API.
There are already two Akismet API implementations for ASP.NET, but they are both licensed under the GPL which I won’t allow near Subtext (for more on open source licenses, see my series on the topic).
So I recently implemented an API for Akismet in C# to share with the DasBlog (despite the bitter public mudslinging between blog engines, there is nothing but hugs behind the scenes.) folks as part of the Subtext project, thus it is BSD licensed.
You can download the assembly and source code and take a look. It is also in the Subtext Subversion repository.