Redundant binary signed-digit

[gilianzz]

this is quite interesting, although I recommend you look up nega bases, such as; nega-decimal and nega-binary.

they basically make negatives super easy.
http://en.wikipedia.org/wiki/Negative_base

I've made a negabinary adder on minecraft aswell, unfortunately they are quite slow and bulky, and the amount of positive vs negative numbers is not symmetrical, for example with 4 bits you have a range of -10,5