Zsh
Mac OS by default uses Bash as Terminal emulator.
Why should you use Zsh?
First, the bash version that Mac OS ships is pretty old (just take a look at bash --version) and zsh offers a lot of nice features and extensions that makes your life a lot easier.
See "Why Zsh is Cooler than your shell" for more on this topic.
Installation
Use brew to install it
brew install zsh zsh-completions
Oh my Zsh
This is an very useful compilation of addons and themes for the Zsh.
Install it with:
sh -c "$(curl -fsSL https://raw.github.com/robbyrussell/oh-my-zsh/master/tools/install.sh)"
By default the robbyrussell theme is used, but there a lot of different ones to choose from (Change it in the .zshrc).
Configuration
Similar to the bas there is a hidden file (starting with dot) in your home directory called:
.zshrc
In it you can set up you can set the theme and other options for the zsh. Or setup any other task which should get executed, when you open a new instance (like aliases, exporting editor etc.)
Switch to Zsh
Change your default shell with
chsh -s /bin/zsh
Alternatives
Bash, Sh, Kornshell