Rubik's puzzles
One of the biggest brain teasers ever made the Rubik's cube has sold over 400 million copies. But most of those have probably never been solved! Rubik's cube was so popular the it spawned a host of other puzzles as well. There's the mini or Pocket Cube which is a 2x2x2 version of the original cube and is a great first step before attempting the full Rubik's cube. There's also the Floppy Cube which is just one layer of the original cube and therefore drastically simpler. And then there is the Domino Cube which is two layers of the original cube and somewhere in between the Floppy and Rubik's cube in terms of difficulty. Each presents its own challenge and all the solutions plus others are on the website.