The comment spam filtering software used on this blog is Spam Karma. Three weeks ago, Dave, the author of the WordPress plugin, announced that Spam Karma has now gone GPL v2. I have been using this plugin exclusively for my comment spam since this blog has been up and running.
Prior to the plugin going GPL, Dave held the license as proprietary, keeping anyone from repackaging his hard work and making money off of it. However, he also stated on the site that if you asked, he would probably give you full permission to modify the code, and make amendments as necessary. Due to this disclaimer, I installed in on my blog. However, the reason for Dave making Spam Karma GPL has nothing to do with Free Software, but more to the fact that he is no longer supporting the utility, and hopes by making it GPL, the community will pick it up, and run with it. I hope so too, because while I'm a major proponent of Free Software, I won't run outdated, unsupported software either.
So, if you know PHP, and have a desire to help with a project, pick up Spam Karma. I know I would benefit from your efforts.