A website consisting of “hacking” challenges of varying difficulties. It’s been around for a long time now and I remember having some fun with it back in the day when I was younger.
The site still works and you’ll need an account to tackle the challenges.
Of course I remember notpron! I even thought about it when making the post, but I couldn’t remember the name! Apparently notpron is still online and can be played, so I know what I’ll be doing now :D