I have always been fascinated by what hackers are capable of doing. I have always wanted to be an ethical hacker some time in the future, but the problem is there is not much courses available on "Hacker" nor there is books available on "How to become a hacker in 21 days?".
Last night I got some free time to browse about hackers and came across this hackthissite. It is a pretty good site with nice challenges on hacking. It was very interesting to solve those questions. I wanted to try solving some more puzzles from that site to keep my brain working/thinking in different ways. Hackers normally tend to think way too out of the box. I would suggest any developer looking for something challenging to take up the basic challenge mentioned in their site.
Some other sites worth mentioning are
ngsec - http://www.ngsec.com
Try2Hack - http://www.try2hack.nl
HackMe @ Elderson - http://hackme.elderson.net
Mindlock Security Challene - http://mindlock.bestweb.net/join.php
Zebulun Challenge - http://www.cyberarmy.com/zebulun/
Root Hack - http://www.roothack.org/
DataFort - http://hack.datafort.net/
HackNull - http://hacknull.com/
Digital Evolution - http://wargames.unix.se/
Happy Hacking :)