Gentoo/Alt is a Gentoo Linux project created to manage porting the Portage framework and other features to other operating systems, such as Mac OS X and the free BSDs. Gentoo/Alt was set up by Pieter Van den Abeele and Daniel Robbins after Pieter Van den Abeele founded Gentoo for Mac OS X.
Gentoo for Mac OS X
Gentoo for Mac OS X was the first non-Linux project of Gentoo and focused on making the Gentoo experience available on Apple's operating system by introducing the Portage system as a separate entity. This was roughly similar to Fink and MacPorts, but it used Portage instead of a Debian-like or Ports-like system. Later on, Gentoo for Mac OS X was made a subproject of Gentoo/Alt. Currently, the project is no longer active, because its prime assumption of using and not modifying the host OS appeared not to be realistic and eventually broke most packages or made them hardly maintainable. Gentoo for Mac OS X has been superseded by Gentoo Prefix, which is currently what Gentoo offers to Mac OS X users.
Gentoo/*BSD is a subproject which covers ports to BSD-derived operating systems. Currently it consists of three sections: Gentoo/FreeBSD, Gentoo/NetBSD and Gentoo/OpenBSD. The Gentoo/*BSD project is an umbrella to better classify these three subprojects, which often have a lot of similar issues: in practice, most of the decision-making and development is handled in the Gentoo/Alt project or in the subprojects themselves.
Gentoo/FreeBSD is a Unix-like operating system developed by Gentoo Linux developers in order to bring Gentoo Linux design principles such as Portage and the Gentoo Linux baselayout to the FreeBSD operating system.
Gentoo/NetBSD is a project to provide a GNU userland managed by Portage with a NetBSD kernel. The project was started by Damian Florczyk. Currently only the x86 architecture is targeted and the system as a whole is in an incomplete state.
Gentoo/OpenBSD is a Gentoo/*BSD subproject to port Gentoo features such as Portage to the OpenBSD operating system. It was originally started by Grant Goodyear and is currently on development hiatus. An ISO image based on OpenBSD 3.8 is currently maintained by Karol Pasternak and can be downloaded from the project's web site.
Gentoo GNU Hurd
The youngest member of Gentoo/Alt is the Gentoo Prefix project. It emerged from the Gentoo for Mac OS X project, which suffered from multiple practical problems initially. It allows installation on Mac OS X, Solaris, FreeBSD and other Linux distributions. The core principle of Gentoo Prefix is that installation of packages by Portage are done in an offset, thereby leaving the host OS unmodified. As a side effect, administrative privileges are not required.
Gentoo/Interix (eprefix) is a port of Gentoo that runs atop the Interix Subsystem for Windows which is also known as Microsoft Windows Services for UNIX (SFU) or Subsystem for Unix-based Applications (SUA).
A result of the Gentoo/Interix project is the ability to install and use the Portage system to emerge native Windows applications (requires Visual Studio, 2008 Express Edition will do too). However, this feature does not support the wide variety of packages supported by other platforms (including Interix).
Mac OS X
An unofficial port to the Solaris operating system, "Portaris", was announced around 2006, but never got into a public state. It has been superseded by the Gentoo Prefix project.