Though their website comes with a formidable handle mathtwitterblogosphere (MTBoS pronounced mitt-boss) don't let that stop you from joining.
Dan Meyer wrote: "File this [MTBoS] as Reason #437 I'm proud to be a part of this enormous professional community. link"
Today (October 6th) begins 8 weeks of "Exploring the Mathtwitterblogosphere." Join up and follow this crash course.
Also on Tuesday nights you can join the Global Math Department for weekly presentations about math for teachers by teachers.
This week (Tuesday, October 8th 9pm) Karim Kai Ani (of Mathalicious.com fame) will be leading the conversation. You are all invited!