I recently came across Typealyzer. It is a simple concept. Paste the URL of a blog into the box that appears and it gives you an analysis of the personality type or the author and a chart of what sort of brain activity was going on e.g. logical/mathematical, intuitive etc
Apparently it references some sort of database and applies an algorithm to spit out these results and for the couple I tried it seemed to be on the money. I don't know how much store I put into a lot of these left brain/ right brain things but it could be a great way to engage your students in metacognition and motivate their writing. What a great discussion point if someone who seems to be all hard-nosed and logical seems to be writing an emotional blog.
The creators point out that it is not necessarily profiling the personality of the writer, but of the voice that they are using in their blog. You could well have multiple blogs that each have a different assessment. I have included my brain chart for this blog... hmmm... do you think it matches a science / maths teacher?