There are numerous Mathematics blogs out there!
Use Mathblogging.org to see recent posts from an extensive collection of blogs.
The Edublogs Directory has a Math and Science Section, like Mathsblogging.org it is possible to see when each blog was last updated.
A site always popular with my students who like the very clear notes available is mr barton maths.com. Mr Barton also has a blog (thank you for the mention, mr barton!) Note the link to the lovely set of 30 puzzles in his recent post – Lovely Puzzles.
A blog I have mentioned previously is ‘Teaching AQA GCSE Maths’ from the Longman Maths publishing team. Obviously this is of particular interest to teachers teaching the AQA specification, however they sometime provide free resources of interest to any Maths teacher. This recent post for example features a free algebra resource including teachers’ notes. This works very well on the interactive whiteboard. An earlier post gave a link to a free book – ‘Street Fighting Mathematics’ by Sanjoy Mahajan. The post includes a link for a free pdf download.
Two more of my own favourites include Dan Meyer’s blog and Guillermo Bautista’s very well organised ‘Mathematics and Multimedia‘; this includes an extensive collection of clear tutorials on GeoGebra. For further links on Geogebra see this earlier post.