This is a device I got in December 2010, for IIRC $130. It detects tension ("grrr"), eye movement, and some other brain waves. In my experimentation with it, I found the brain wave modes hard to control, while the others were easier.

There was another model I saw that was $300, but I can't remember the name. The amazon page should get you started on finding related hardware.

Note: I haven't actually used a BCI with Dasher, it was just an idea I had for hands-free typing (and possible improvements in costs over existing tools for the disabled).