This site will let you practice your l33t skills at haxoring.

The site is pretty self explanatory.

http://www.overthewire.org/

From the about page:
Mission:

OverTheWire.org / PullThePlug.org is a non-profit organization aimed at nurturing the growth and development of the information security field through community-wide research and development projects, lectures and wargame servers. Rather than taking the usual approach of hands-off documentation, we emphasizes education and development as a community effort.


There are also some guides and videos to help you along the way. There is plenty to do so have a look around.