I saw these guys covered on Slashdot today. I remember a similar tactic used last year that didn't last too long for some reason, but was apparently quite effective.
There are ethical issues on DDoSing spammers, but having thought about it for six seconds, I don't have a problem with it -- assuming you're using your own bandwidth to do it and not your zombie army of trojaned Windows drones :)
Speaking of Windows, they don't have other versions of the software available, but they have provided the source code (good on them). Hopefully we should start seeing some meaningful ports soon.