Ethical Hacker just launched a new challenge. This one however is a little different since you need to buy the book Daemon to be able to solve it. The book itself has some good reviews, but still, this makes it out of reach for a lot of us :-(.
And the second challenge (which is unfortunately already over :-)) is from Didier Stevens. In fact it isn't even as much a challenge as a puzzle. Still worth a look at through, especially the solution method described in the comments.