Zahada: The Most Challenging Riddles Online
mcgov.co.uk
mcgov.co.uk
Not trying spoil it for anyone else though so I won't give details. I will say if you think you are on to something and it just doesn't work as a correct answer, keep trying. You might be right but they thought of something else for an answer.
qvq lbh guvax bs gur jbeq gung zrnaf srznyr furrc?
(Though I guess it wouldn't have worked with the "double / single / black, blue, and grey" clues anyway...)
Edit: I'm an idiot :D
Hint: look at the source code to solve many of these.
"Please Note: Zahada is designed for Mozilla Firefox - some riddles may not work in other browsers"