Haven't done one of these for ages, but was reminded when I accidentally found the first ever one: Friday Night Free For All on Sir Humphreys' on June 10, 2005. More than seven years ago now.
Since then, a number of other blogs have taken up the open thread concept, KiwiBlog's General Debate being a prime example.
I think they are a good idea for places where commenters gather. We're not so busy in that way anymore, but for blogs that are busy, it gives people an opportunity to talk to each other about anything rather than having to loosely stick to the topic of the post. It builds community if it's done well.