I've grown tremendously in the last six months as a teacher. Through Twitter I've found like-minded educators intent on improving their craft. I started by following people I knew to be involved in the math education world--the rock stars of math education. Quickly, I found looking towards a 'hero' is not the best use of the tool. The beauty of Twitter is the sharing that happens throughout the math community not just through the sages.
#MichEd - Wednesdays at 8pm
A gathering of Michigan Educators who share best practices and encourage each other to grow as teachers.
The Math Twitter Blog-o-Sphere community. Math teachers from around the world (literaly) who freely share lessons and ask questions. If you're looking for collaboration as a math teacher, this is where it's at!
#Alg2Chat - Mondays at 9m
I devoted my time this last summer to reinventing my Algebra 2 class. It's not that it was a bad course before, it's just that there weren't a lot of engaging activities and experiments. Through the #MTBoS, I found other teachers who were intent on developing their own Algebra 2 courses. Here's an awesome idea for M&M catapults from @HHSmath. Guess what I'm going to do this year when we cover exponential equations.
Once you've gotten your feet wet with Twitter, I suggest using Tweetdeck. Once you do, there is no going back.