Recs.
Updated
a free Unix-like operating system descended from Research Unix via the Berkeley Software Distribution
SpecsUpdate
Pros
Pro Great philosophy
FreeBSD won't change everything from a version to another, it tries to keep and maintain old tools as long as possible, and won't replace half the system every two versions like GNU/Linux distributions such as Ubuntu, Arch or Fedora does. everything is stable and also pretty minimal. FreeBSD is designed for performance.
Pro Powerful ports collection
Ports provide a wide collection of software which are easy to build, install and modify. They contain recipes and patches to build various software, so you can simply run "make && make install" to build and install the software. You may also keep local patches and it would be picked up automatically. It also allows you to use the latest software even if you are not using the latest version of the operating system.
Cons
Con Bad third-party application support
Even most software that's portable to Linux will have a hard time running on FreeBSD. Sure, it provides binary compatibility with Linux, allowing users to install and run most Linux binaries on a FreeBSD system without having to first modify the binary, however, some Linux-specific operating system features are not supported under FreeBSD (for example, Linux binaries will not work on FreeBSD if they overly use i386 specific calls). You're stuck with mostly running only the free software.