Completion of various challenges require experience in web security. SQL injection, apache, exploiting PHP, etc.

http://hax.tor.hu/

Level 1. Make a nasa.gov URL display a text of my choice
Level 4. IP address is 72.14.207.99. What is geek that points to it?
Level 6. Let's see you do some easy SQL ninjitsu
Level 7. snifflog.txt - ngrep format
Level 13. PHP with source - needs exploiting and/or o-o-t-b thinking
Level 15. download.com's uptime
Level 16. root:hsmfs;[email protected]