fish is a friendly command line shell (Friendly Interactive SHell) for UNIX-like operating systems such as Linux.
Command completion
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Fish gives you very good default completions. For example it will complete hostnames from your .ssh/known_hosts. For example, if you want to ssh into your account on host foo.example.com and you have connected to this host before, you can type
ssh foo<tab>
and Fish will let you complete the host name from there.
Fish has a different take on completing commands from your history than other (eg. Bash) shells. Instead of having you type cryptic key combinations like cmd-R which takes you to an emacs-like minibuffer, Fish lets you use the arrow up-key and will complete the command from your history based on what you typed.
$ cd /path/to/silly/directory
$ make
$ cd
$ cd /<up-arrow>
The last line in the example above will let you jump through your command history and match all commands you have typed that begin with "cd /". If the first match wasn't what you were looking for, press up arrow again and Fish will keep trying.
Fish also has builtin command completion for a lot of applications; after installing fish just try hitting tab after a command name and see if it doesn't suggest something relevant for you.
Get help
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To get help with fish, type help at the command line - optionally followed by a command or function you want to get help with
Go fishing
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The Fish shell can be found at http://www.fishshell.org/