HackThisSite.org is a free, safe and legal training ground for hackers to test and expand their ethical hacking skills with challenges, CTFs, and more.
I used the site to learn about ethical hacking, the various holes and errors that you can make when setting up a site.
As you can see by the screenshot above, there's some coding skills, detective work and trial and error needed to advance.
Each level gets more and more difficult.
I had to stop when my health deteriorated, but I did manage to reach level 5.
If you look at the mirrored Wikipedia page above, you'll see that there has been a rift between the founders over the ethics of the site.
It is lots of fun. Most stackers will whizz through the levels like a breeze.
So, upgrade your cowboy hat for an even whiter (cowboy) hat, share your experiences, try the levels, but don't identify yourself too much - after all you are meant to be getting better opsec skills, not worse!