Neat number system. A variant of the ternary (base 3) system, balanced ternary uses the 3 trits >-, 0, + (think of + and - as +1 and -1, respectively). So you can count up to ten like this:
+, +-, +0, ++, +--, +-0, +-+, +0-, +00, +0+
Each digit position is 3 times the previous position, just like in the ordinary ternary system.
The advantage of this system is that your 3 digits are enough to represent both positive and negative numbers, with no sign bit! And to flip the sign of a number, just switch +'s and -'s (since the base is odd, a set number of digits allows the same number of positive and negative numbers to be represented, unlike, say, 2's complement binary).
Arithmetic is no more difficult using this system than using regular ternary. In fact, once you're used to it, it's even easier, since like binary, you don't need to memorise any tables...
In the 1960s, there was a Soviet computer called "Setun" which used trits. Wikipedia says it was balanced-ternary (so it must be true).