- CTF Information - Contains practice CTFs and other info
- g0tmi1k.blogspot - Video walkthroughs of tons of the Vulnerable VMs/Software & other great info.
- Microsoft BlueHat
- Top_Coder - Individual Challenges
- Crack Me if you can Password/Hash Cracking
- BSides London Reversing Challenges
- Ethical Hacker Network's Challenge
- Google Summer of Code - could be interesting
- Smash The Stack - Primers: at Art of Exploitation and here
- http://google-gruyere.appspot.com/
- http://www.enigmagroup.org/
- http://www.crackmes.us/
- https://www.facebook.com/hackercup
- http://www.wechall.net/
- http://www.overthewire.org/wargames/
- http://challenge.spider.io/
- http://challenge.constantcontactsecurity.com/ - Starts out extremely easy
- http://sourceforge.net/projects/lampsecurity/ - provides several Capture The Flag challenges
- http://exploit-exercises.com/
- https://pwn0.com/ - Network of Danger and fun!
- Cybersecurity Competition Federation - NSF Grant funded project to help information security competitions and players
- Cybersecurity Competition Federation - Group aiming to help competition players, mentors, sponsors and organizers (Old Link)