Zero is an even number.
An even number is a number that can be divided by 2 with no remainder.
0 ÷ 2 = 0
There's no remainder, so zero is even.
Like CPhill said, Wikipedia explains that zero is even.
Also, many other math resources say the same.
If you don't believe me, here's some links:
http://www.icoachmath.com/math_dictionary/even_number.html
http://www.mathsisfun.com/definitions/even-number.html
http://www.factmonster.com/ipka/A0876700.html
http://mathworld.wolfram.com/EvenNumber.html
http://www.mathwords.com/e/even_number.htm
...and there are others! :)