I like this comparison between Linux and FreeBSD:

Although FreeBSD and Linux are close cousins with a considerable number of similarities under the hood, some major differences separate them. FreeBSD is tidy, self-contained, and well-organized. All the pieces form a harmonious whole — a place for everything, everything in its place, and pretty much just one way to do anything.

Linux is more like a barrel of monkeys — loud, messy, chaotic and very busy. Every monkey thinks she knows the best way to accomplish a particular task, so there are always several ways to do any one thing. The Linux world is faster-paced and more diverse, but sometimes a person just wants a nice calm computer on which to do work without all the drama.

FreeBSD is the most popular of the open source Unix operating systems. It’s a top-of-the-line genuine Unix, and it powers many of the world’s most demanding Web servers. Because it is secure, stable and easily manageable via its Ports system of package management, FreeBSD is a popular platform for servers of all kinds. FreeBSD also runs Linux binaries, so you can run pretty much any applications you want. Its hardware support is not as robust as Linux, however, so you do have to shop a little more carefully.