Whenever David Wees comments on one of my blog posts it's always insightful and it challenges my thinking. David brings those same qualities to his blog posts about mathematics instruction and using technology in the classroom. Read David's recent post about raising mathematicians to get a good sense of what he's about.
Colleen Young's Mathematics, Learning and Web 2.0 is a good blog to subscribe to for practical, do-now, mathematics instruction ideas. When you visit her blog make sure you check out the "I'm Looking For" and "Wolfram Alpha" tabs.
Mathematics and Multimedia written by Guillermo Bautista is the blog that I usually recommend when people ask me for recommendations for learning about GeoGebra. Guillermo's blog has a goodcollection of GeoGebra tutorials for both beginning and advanced users of GeoGebra.