Labs
- IRC
- Nessus
- Netcat
- Nmap
- Shellcode
- Buffer Overflows
- Buffer Overflows II
- Metasploit Basics
- Heap Overflows
- Format Strings
- Web Attacks
- Password Cracking
- Firewalls
- Stagers
- Password Cracking 2
- Session Hijack
- Reverse Engineering
Always-on Online CTFs
- io.smashthestack.org
- HackThisSite
- Matasno/Square CTF (beginners level)
There are plenty of other annual CTFs: Ghost in the Shellcode, Defcon, iCTF, and others worldwide.