Late in 2008 I wrote a post about a huge drop in spam.
In that post I was amazed by a period where there was almost a 50% drop in spam. Recently I have measured a drop of more than 80%.
Again I turned to Brian Krebs for an explanation.
Given that my spam dropped in late August/early September I think the key point is in Brian's update near the bottom. In summary, two major spam botnets have been slowed down.