There was one site that had something like 100(?) brain teaser exercises - I suppose you could also call them puzzles - which became successively more difficult as you moved on (ie, hours to figure out just one of them). Each was distinct and challenged you in a different way. I only used the site once, around the early 2000s, but I forgot its name and never found it again.
When I look at the Archives on Project Euler, the same general idea is there but the site I miss wasn't programming focused. I seem to recall a giant red, blue or green background and the puzzles weren't just text-based, but visual.
