Zsh

(Z shell) is an interactive login shell that can also be used as a powerful scripting language interpreter. It is similar to bash and the Korn shell, but offers extensive configurability, powerful command-line completion, file globbing, and spelling correction.

Emerge
Install :

app-shells/zsh-completions
Emerging enables auto-completion for arguments of commands, which is one of the advantages  has over other shells:

app-shells/gentoo-zsh-completions
Emerging enables Gentoo specific auto-completion for arguments of Portage and other Gentoo commands:

When installing this package be sure to add the following to the respective files:

To enable a cache for the completions add:

Invocation
Upon running for the first time as a new user, you will be greeted by a basic configuration dialog. The setup process can be skipped by pressing. If the setup process is skipped can be setup manually.

Setting as the default shell
To make the default shell for a user, run:

File
's main configuration file is located in each user's home directory at. Reload this file in running shells for the changes to take effect:

oh-my-zsh
The zsh community created numerous tweaks, the easiest way to acquire them is to install oh-my-zsh framework. It contains handy plugins and eye candy themes, and makes their configuration very easy. However you should consider the security risk involving running code outside of Gentoo developers jurisdiction.

External resources

 * Official introduction
 * Official User's Guide
 * Documentation in various formats
 * FAQ
 * Wiki
 * Lovers
 * GRML Configuration
 * oh-my-zsh
 * Sorin Ionescu maintained oh-my-zsh fork now called prezto - The configuration framework for
 * ArchWiki page