Defensio, the anti-spam service, offers an API that can be integrated into blogs and web sites to fight spam comments and other unwanted spam (details at our Defension API profile). The service appears to be run by a small team, and is similar in purpose to Akismet, the much larger spam filtering service from Automattic (which powers WordPress.com), who also offer an anti-spam API.
The Defensio API lets you submit text entered by a user which is then analyzed by their service and returns an indication of the likelihood that the text is spam. Additional functions let you help train the Defensio database by identifying false negatives (cases where Defensio failed to identify actual spam) and false positives (cases where Defensio incorrectly considered a text to be spam).
The REST-based API supports data in XML or YAML. (Don't know YAML? It stands for "YAML Ain't Markup Language", a human friendly, programming language independent data serialization format). Defensio also provides OpenID support, API documentation, an FAQ, community forums, and the Defensio blog.
Defensio plugins are available for a number of popular blogging applications including WordPress, Korea's TextCube (recently acquired by Google), and the ExpressionEngine CMS. They also offer language specific wrappers for Java, PHP, Ruby on Rails, Python and .NET.