In case you haven’t come across “The Toughest Developer Puzzle Ever” [sic] then give it a go. It’s by no means the toughest puzzle I’ve ever done, and some parts of it are embarrassingly lame, but it’s addictive and very frustrating. In fact I’ve pissed away the best part of my holiday weekend doing it. Give it a go if you have a geek gene and time to spare.
The only tip I’ll give is to remember that this is written by .NET monkeys; if you find yourself doing anything even remotely complex in trying to answer the questions, you’re almost certainly on the wrong track.