Nuttx is a real-time embedded operating system (RTOS). It has a small footprint that is usable in micro-controller environments. It is fully scalable from tiny (8-bit) to moderate embedded (32-bit) systems. It also aims to be fully compliant to standards, to be fully real time, and to be totally open.
MINIX is a free UNIX clone that is available with all the source code. Due to its small size, microkernel-based design, and ample documentation, it is well suited to people who want to run a UNIX-like system on their personal computer and learn about how such systems work inside. It is possible for a person unfamiliar with operating system internals to understand nearly the entire system after just a few months use and study.
Get Patches is a small Perl tool for the retrieval of Solaris patches (IA and SPARC). A rather ugly shell script for applying the downloaded patches is included since you might not always want to apply the patches automatically. Get Patches requires the Sun patchdiag tool and Perl (including the LWP module).
Loonix is a custom Linux distribution meant for server applications. It comes with only the latest up-to-date programs and applications, all specially configured for optimal performance and ease of use. Programs are neatly organized in structured directories, and strict security rules are in place for sensitive configuration files and other data.
Briefly, FreeBSD is a UNIX operating system based on U.C. Berkeley's 4.4BSD-lite release for the i386 platform (and recently the alpha platform). It is also based indirectly on William Jolitz's port of U.C. Berkeley's Net/2 to the i386, known as 386BSD, though very little of the 386BSD code remains. A fuller description of what FreeBSD is and how it can work for you may be found on the FreeBSD home page.