A very interesting challenge is Try2Hack
There are different levels and your goal is always to find out a password.
It's getting harder from level to level and you learn a lot on the way.
I'm currently on level 6.
WARNING: Do not click on the link above if you want to play the game yourself.
The link is named "Solutions" because it really contains the detailed
descriptions how I solved the levels. It really takes the fun out of it for
you but I need this for my ego ;-)
Please do not send me detailed descriptions how to solve the level where I'm currently on.
I want to find it out on my own. Thanks.