Yesterday I discovered the Tim Tang Test. The creator bills it as "the longest and hardest online puzzle in the world". From my experience of it, I don't think he's kidding.
This thing is frustrating beyond compare.
And worst of all, you can't usually just google the answer when you get stuck - the answers are closely guarded, and the ones you can find online might get you to the next level, but they don't explain why they are the correct answer and so will leave you hollow inside.
Having said that, you are encouraged to use google on many of the puzzles - it even provides a google toolbar on many of them. You will need to do a lot clever googling to get the information you need on some of the puzzles, while others solvable purely by reasoning.... eventually!
Be on the lookout for clues in the mouseover text, the answer entry box, the background music and other, even more obscure places.
There's supposedly 250 levels. I am on level 23. So far, no-one has completed it. Few have even made it into the 100s.
So far, Level 6 was the most frustrating, and I'm still not quite sure how I got the right answer.... which is frustrating in itself!
For those of you who dare, you can find the Tim Tang Test here.
Don't say I didn't warn you!