Portage is a
package manager which was originally developped for
Gentoo, but is now also used by ChromeOS, Calculate, Sabayon and Funtoo Linux.
Because Portage is integral to Gentoo-based distributions, there's no need to install it separately on them.
Misc
Portage is based on the FreeBSD Ports collection.
The Compile system (which is used in GoboLinux) is somewhat similar to Portage.