TypePad AntiSpam is being billed as a free, open source anti-comment spam filter. It's already built into TypePad blogs (has been for more than a year now) and can be installed as a plug-in for MovableType and WordPress platforms.
Much like conventional heuristic e-mail anti-spam filters, TypePad AntiSpam's engine gets smarter the more it's used and the more comments it evaluates.
I'm not sure how Six Apart is going to monetize this, or even if it has to. Here's what Anil Dash said on the Six Apart blog post:
The more different implementations of spam-fighting technology that exist, the more complex and challenging (and expensive!) it becomes for spammers to keep attacking our communities. At the same time, we want to make sure our economic incentives at Six Apart as a business are aligned with the best interests of bloggers, so that we feel the pain and cost of spam just as you do.
Maybe Six Apart is doing this to be a good netizen. But it also may siphon cash flow from its competitor, Automattic. Automattic charges $50 a month for the "enterprise keys" to its Akismet anti-comment spam filter, and $5 a month to anyone making "mad paper" from their blog. (As defined by Automattic, "mad paper" is income of more than $500 a month.)
If Akismet proves to be a "much" better filter, it won't have much to worry about. But if TypePad's engine comes anywhere near as effective, Akismet will also be very free, very soon.