Negabinary

Negabinary (also known as base -2) is a negative positional numeral system with a radix of -2. It is the negative counterpart of binary and as such similar to it in many ways.

Usage
Negabinary has seen some use in the old computers of BINEG and UMC. It is the only negative base which has seen real world use. However, while negabinary can represent a number without a minus sign (an extra digit appended at the front of a "standard" binary number) it in turn takes more digits to represent values and it is harder to understand which digit represents what number.

Value of a number
To evaluate a negabinary number, read it as you would in binary, except taking the alternating sum rather than the standard sum at the end. If the number of digits is even, it's a negative number, and the sign in front of the MSD is a minus; if the number of digits is odd, it's a positive number, and the sign in front of the MSD is a plus.