Linux is a clone of the Unix kernel, written from scratch by Linus Torvalds with assistance from a loosely-knit team of hackers across the Net. It aims towards POSIX and Single UNIX Specification compliance. It has all the features you would expect in a modern fully-fledged Unix kernel, including true multitasking, virtual memory, shared libraries, demand loading, shared copy-on-write executables, proper memory management, and TCP/IP networking.
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.
Curie is a very minimalistic, tiny C library. It supports basic I/O, basic networking, S-expression based I/O, signal handling, process handling, and I/O multiplexing. If the target architecture is explicitly supported, statically linked binaries may be as small as 4-20 kilobytes; if not, then it only relies on the host's C library. The library explicitly does not provide a POSIX interface so that it may be used in conjunction with a regular host libc (or without one).
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).
EV-OS is an operating system which runs in real mode on the x86 platform. The system must be booted and run from floppy disk. The user interface is command-line based. Application programs must be prepared for operating system. EV-OS application programs can be developed under the Windows operating system, and a special tool is used to transfer files from Windows to EV-OS. EV-OS is written in assembly and the transfer program is written in C. The downloadable package contains an executable image file, the source code of the operating system, and some installation documentation.