The Slymepit is a phpBB discussion forum mainly centered around discussing the flaws of Freethoughtblogs, Skepchick and Atheism+. The vast majority of discussion goes on in a thread called "Periodic Table of Swearing", rather than taking a traditional forum structure.
The forum itself was started some time after the discussion threads on Abbie Smith's blog were branded a "slime pit" by PZ Myers and other bloggers on FtB. Because they were almost entirely unmoderated in the interests of an open discussion, Abbie was accused of condoning hate speech (when in fact a 'free speech policy' doesn't imply endorsement at all). Eventually, after failing to intimidate Abbie into censoring the comments, Pharyngula regulars launched a campaign to get National Geographic to do it for them.
Due to this threat to the "slime pit's" existence, it was decided it would be moved to its own forum, with the original threads being shut down.