hacker.org is pretty good. Some of the challenges can be brute forced easily, and some have only been solved fully by one person. The puzzles are good, but the

hacker.org is pretty good. Some of the challenges can be brute forced easily, and some have only been solved fully by one person. The puzzles are good, but the real fun is in the challenges. Some of the ones I've unlocked have only been solved by less than 100 people.