OverTheWire – Wargames.

The wargames offered by the OverTheWire community can help you to learn and practice security concepts in the form of funfilled games. To find out more about a certain wargame, just visit its page linked above. If you have a problem, a question or a suggestion, you can join irc.overthewire.org on port 6697 using an SSL-enabled IRC client, or irc.overthewire.org on port 6667 using a regular IRC client.

This is like a text adventure, except its unix/cryptography/C.