NetworkManager

NetworkManager is a network management software for Ethernet, Wifi, DSL, dialup, VPN and mobile broadband network connections.

Kernel
You need to activate the following kernel options:

For Wifi devices enable also the following options:

Software
Portage knows the global USE flag networkmanager for enabling support for NetworkManager in other packages. Enabling this USE flag will pull in automatically:

The USE flags of networkmanager are:

After setting this you want to update your system so the changes take effect:

Also install a frontend:
 * for GNOME
 * for KDE
 * for the command line

There are also some extensions:
 * - VPN connection to a OpenVPN server
 * - VPN connection to a PPTP server
 * - VPN connection to a Cisco VPN concentrator

Configuration
You can now start NetowrkManager:

To start NetworkManager at boot time, add it your default runlevel:

Reload D-Bus so that the NetworkManager changes take effect: