My favorite is 

 man ascii

Simple and so very useful.

![alt text][1]

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Have a look at this website [commandlinefu.com][2].

You can also have a look at these four articles by Peteris Krumins on his blog

 - [Top Ten One-Liners from CommandLineFu Explained][3]
 - [The Next Ten One-Liners from CommandLineFu Explained][4]
 - [Another Ten One-Liners from CommandLineFu Explained][5]
 - [Yet Another Ten One-Liners from CommandLineFu Explained][6]


 [1]: http://i.imgur.com/fc1GI.png
 [2]: http://www.commandlinefu.com/commands/browse
 [3]: http://www.catonmat.net/blog/top-ten-one-liners-from-commandlinefu-explained/
 [4]: http://www.catonmat.net/blog/the-next-ten-one-liners-from-commandlinefu-explained/
 [5]: http://www.catonmat.net/blog/another-ten-one-liners-from-commandlinefu-explained/
 [6]: http://www.catonmat.net/blog/yet-another-ten-one-liners-from-commandlinefu-explained