http://www.cipher-ctf.org/ (available to students only)
Limited to 40 teams.

However, if you are a single person, or if you just want to have a sniff of adventure and therefore join the contest without all the work of being an actual particpant.

In the last CTF, they allow a limited and registered set of interested individuals take part in the contest as third parties. These will not get scored, neither will they host images, but only be allowed access to the VPN in order to attack the hosts.
If you are interested in participating this way, please contact the organizors of the contest directly by mail.

http://smpctf.com

Pre-registration:
http://www.smpctf.com/register.php

IRC: irc.freenode.net #smpctf
For more information:
ctf [at] smpctf [dot] com