freeBSD Linux operating system!!
Since the beginnings, we were inundated with requests to add BSD projects to the site and track them in the same manner as we track Linux distributions. Eventually, we gave in. Although BSD (a direct descendent from UNIX) is not linux operating system(a clone of UNIX), the two operating systems have a lot in common: they are free in both senses of the word, they use many of the same open source applications, and they are highly addictive. This is the reason why the most popular BSD-based OS is included in this list, alongside the top 10 Linux distributions.
FreeBSD has a long and turbulent history dating back to version 1.0, which was released in November 1993. Its development process takes place in two concurrent branches: stable (production branch) and current (new technology branch); these can be roughly compared to Debian's stable and unstable branches. However, unlike Debian, the developers of FreeBSD make regular full releases of both branches, which they tag as "RELEASE". Thus, you might see FreeBSD releases such as 4.9-RELEASE and 5.2-RELEASE, released within a short period of each other, but it doesn't mean that FreeBSD 5.2 is newer than FreeBSD 4.9; it just means that the release with a higher version number is a development release with some experimental features, not recommended for production use. At some point in the future, a FreeBSD 5.x release will be declared stable and a new current branch, tagged as FreeBSD 6.x, will be opened.
Although FreeBSD has a well-deserved reputation for being a fast, high-performance and extremely stable server, it can be used as a desktop system just as well. Installation is not difficult, certainly no more difficult than the installation of Slackware Linux. If you come from the Linux world, you will be pleased to learn that the vast majority of open source applications you've come to know and enjoy (e.g. XFree86, KDE, GNOME, Apache, and even some non-free software, such as the NVIDIA accelerated driver or the Opera browser) also work on FreeBSD. Besides binary packages, FreeBSD (like most other BSDs) has an extensive collection of "ports" providing users with an easy way of compiling every package on a host system. This is probably the main reason why many users prefer FreeBSD over Linux.
Pros: Fast and very stable; has excellent documentation, availability of "ports" for compiling software applications locally
Cons: Tends to be behind Linux when it comes to support for newer hardware, limited availability of commercial applications
Software package management: Binary packages and source-based "ports" (TBZ)
Free download: Yes