A brief update on Planet Antispam...
I've just added MailChannels' Anti-Spam Blog. Now -- in the interests of disclosure -- I'm a member of MailChannels' Technical Advisory Board. However, that didn't affect this -- their blog has had consistently good, interesting posts dealing with anti-spam-related topics, and without too much plugging of their own products. ;)
Also added recently:
If you know of any other good email anti-spam-related blogs, drop a line in the comments here. (Note that I'm trying to keep it email-related, however, so we're not covering web-spam.)