How would you use a blog in your subject area?

Blogging is free, easy and can become quite addictive - how many times this week have you updated your facebook profile ha?  When I first starting using them I was quite blinkered, in that I only thought that they'd be good for 6th form students, or for certain subjects.  However, they are just as applicable to KS3 and KS4 and, I'm hoping that with your input, together we'll come up with a whole host of fun, interesting and educational ways to use a blog as part of our regular teaching.  So, as experts in your own areas tell me how you would use a blog.  Let the blogging begin...