Over the years I have been collecting links that relate to mathematics and sharing them within my school board. A few years ago I started adding them to a social bookmarking site. They can be found here https://delicious.com/davidpetro (there are almost 2000 now). Somewhere along the way I started to just create these weekly math link emails and sent them to my school board. Over the last few years started to turn those emails into blog posts.
One thing of note, I teach in Ontario, Canada. So all of the curriculum tags I use refer specifically to our math courses. Sorry about that rest of world. That being said, I make a point to share a lot of what math teachers in Ontario are doing and the good news is that more and more of them are starting to share online.
If you have a blog or know of a blog that I should be looking at, drop me a line via email or Twitter.
Other blogs I contribute to
Engaging Math - my teaching partner and I have started to share all of the activities that we have developed for grade 7-12 math
Found Data - finding data sets that can be used for teaching statistics and data management that either help teach a concept or are of interest to students (or both)
Don't Fight the Tech, Use the Right Tech - I sometimes contribute to this blog which is about using technology, in general, in classrooms.