Very interesting Wired article about extending or adding senses to the human body: I very much like the idea - and would love to try it - of the belt of vibrating units always indicating the direction north called feelSpace. It seems that the body is able to learn to subconsciously translate the vibrations around the waist into a new 'orientation sense'.
In a somewhat similar direction there is a fascinating talk by Quinn Norton (slides) about functional body hacking - which includes for example the (not recommended) implant of a small magnet into a fingertip - enabling a completely new sense of our environment. This also sounds pretty cool - I wonder whether one could get a similar new feeling by not implanting it, but always attach a small magnet to a fingertip?