Today I stumbled upon metaoptimize.
I am always looking for new blogs/communities/forums an this one seems very active and very in-depth.
There is a stackoverflow like Q&A section with many many questions and comments on many machine learning topics.
There is a lot on probabilistic modeling there which I find particularly interesting. I haven't seen any other community like this.
The reason I found this community is actually that this blog was linked from there :)
There is a thread on machine learning blogs that gives many more than I was aware of...
I did not have the time to look into all of them but I will definitely try and follow them all :)