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.
ISDN Router allows you to convert old hardware into a secure masquerading ISDN router, including caching nameserver, IP Port Forwarding, and on-demand channel bundling. The system fits onto a single disk, and users can change the configuration through a simple menu-based system (on the console or over telnet) and store it permanently on the disk.
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.
The OpenBSD project produces a free, multi-platform 4.4BSD-based UNIX-like operating system. Its goals emphasize portability, standardization, correctness, proactive security, and integrated cryptography. OpenBSD supports binary emulation of most programs from SVR4 (Solaris), FreeBSD, Linux, BSD/OS, SunOS, and HP-UX.
Papillon is a security module designed for the Solaris Operating Environment. It provides security mechanisms and protections that improve the overall security of the system by adding new functionality to the kernel such as a restricted proc, chroot environment protections, secure STDIO file descriptors, restricted symlinks in /tmp, setuid protections, and more. In the current version Papillon supports Solaris (8, 9, and 10) and OpenSolaris running on x86 and SPARC architectures in 32- or 64-bit mode.
Contiki is an open source, highly portable, networked, multi-tasking operating system for the Internet of Things. Contiki includes a multitasking kernel, a TCP/IP stack and a set of application programs, and a low-power radio communication stack. It is written in C and designed to be very small: it runs comfortably in a few kilobytes of RAM.
MirBSD originated as a patch set against OpenBSD-current, an ultra secure operating system and NetBSD derivate, and has since also incorporated changes from NetBSD, a 4.4BSD-derived ultra portable operating system. It features bugfixes, code removal for the sake of simplicity, and feature enhancements over stock OpenBSD as well as a much more up-to-date GNU toolchain, careful integration of patches from other projects (such as KAME), and many improvements. It works on the Intel Pentium and some 80486 machines with more than 32 MiB RAM and the SPARC, and a port to the PowerPC Macintosh is in preparation.