Paludis

Paludis is an alternative package manager to Portage written in C++ and bash. It has C++, python, and ruby bindings. It's fast and stable enough for everyday use.

Installation
See Paludis/Installation.

Usage & about this wiki article
Paludis is a very professional and well thought-out project. It is designed elegantly and most of its behavior is very logical. That said, the topic of the #paludis IRC channel reads: "The FAQ is not a substitute for reading the full documentation.". Similarly, this wiki is a guide to get you started, but it is in no way a full documentation of Paludis.

Overlays
Read the Overlays article.

Genlop replacement for Paludis
In Paludis the command that has the same usage as Portage's Genlop is called Plop and can be obtained from the THE overlay. The syntax and usage of Plop is identical to that of Genlop to make life even easier.

Cleaning up clutter in
When your config files get cluttered with obsolete, repeating etc. settings (e.g. user masks, unmasks, keywords), there is a script (somewhat similar to eix-test-obsolete) that you can use: You can get it from the config-decruft Git page.

Note that you need the ruby-bindings use flag on paludis for the script to work.

Use in other distributions
Paludis, while originally written as a replacement to Portage, is used in several distributions. As said, it's the default package manager in Exherbo, and is also known to work in non-Linux distributions like FreeBSD