Here some informations about various sensor-types and how to connect them to a midibox.
Keep in mind that most of sensors are analog , so they require an AINSER Module to be plugged as inputs to the Core Board.
Sensor-Wiki maintains a free list of possible sensors you could experiment with . For example the picture on the right is a EEG Active Electrode from the OpenEEG Project. A mind-controlled midibox ? who said that ?
If you're interested in working with sensors, you might also want to check out the ACSensorizer-Midibox project that AudioCommander leaded.