Blue Ice
This game is impossible. The game itself is so rediculously hard, that words can barely describe it.
It's an old point and clicker that you basically have series of puzzles and whatnot. Think you're doing good? Well, entire puzzles and riddles are simply red herrings. For years and years there was NO ACTUAL ENDING. See, it was made for a contest with prizes. Well, the fact is, people would have just hacked to the end, so he never put one in! I say for years and years because he finally walked players through to get to the end just recently, so in essence there is now an END, just not an ending.
But the whole game, besides it's awesome mind bending weirdness, toughness, and borderline incompleteness has a really cool feel to it. If you have a phd in cryptography, I highly suggest you give it a shot without a guide. I believe you will also need about a decade of free time as well.