I agree with the previous posts, just to add some other practical things.
- All of the *BSDs use what is called a ports tree, which is a system that uses the make utility to handle software installation/deinstallation. Some responsible developer (maintainer) for each software piece, maintains it's "port", that is, writes a Makefile that usually configures, patches and compiles the original source code of the software in a way that is supposed to be tested in a large extend and found to work in every *BSD installation. It also marks the dependencies for each software, so that they can be resolved/installed automatically with no trouble. In practice, that works superb (MUCH better than using precompiled packages in package systems such as RPM). Every of the *BSD has it's own port tree which is usually built daily (as chances are that every day some of the maintainers will have an updated port for some software).
- Of all the *BSD FreeBSD has by far the largest port tree (over 13.000 applications, nicelly sorted in categories). It also has the largest support/community. If something does not play in FreeBSD, chanses are it will not play in any *BSD. FreeBSD's strenghts are it's performance, configurability, support and maturity. It also has some other more specific original pieces of software to brag about, such as the commendable Virtual Memory subsystem, an excellent Filesystem, the linux compatibility layer, and what is considered by many the best TCP/IP stack implementation. Some of the largest internet servers are based on FreeBSD.
- NetBSD is equally matured to FreeBSD, they are developed paralelly since almost their begining. NetBSD's priority is to offer maximum portability, that is as cybersorcerer said, to be able to work flawlessly in every platform. Of course that is not much of a concern if you intend to use a x86 machine. In the past, almost in everything, NetBSD came second behind FreeBSD regarding performance. Still, in it's recent version, 2.0, there has been major improvement (such as the addition of native thread support, with an inteligent method they named Scheduler Activation) and now it has been found to be slightly faster than FreeBSD 5 in most things! NetBSD does not have such a large port collection, but it is big enough counting more than 5.000 software titles!
- OpenBSD is a unique BSD flavour. It began as a project by Theo de Raadt when he seperated from NetBSD project team after a development-course disagreement. It must be said that Theo is considered by many one of the bigest programming geniouses. Theo started the OpenBSD project somewhen around 1993 with ultimate and absolute priority, security! In it's history OpenBSD has set the security standards, it's developers doing anything they could to make OpenBSD as secure as possible. They have audited and debugged exaustingly the OpenBSD code (and they keep doing it) to ensure there are no memory allocation errors that could lead to buffer overflows. They are probably the hardest working team from all the *BSD, and they are well known not to submit to any difficulty EVER (there are many incidents that put serious obstacles to OpenBSD development in the past, but OpenBSD always walked above them stronger than before!). Well, I could praise OpenBSD all day, but I'd better mention some of it's strengths and weaknesses (according to my opinion always).
Good things:
1)OpenBSD is super-duper secure. It aims to be as secure as possible in theory, so it ends up being the most secure in practice.
2)OpenBSD has excellent base support. If there is a problem in it's base, it will be found and fixed by it's team, usually before anyone even suspects it existed. Period.
3)OpenBSD has introduced some superb pieces of software, usually globally recognised and ported. Good examples are the OpenSSH and the PacketFilter software.
Bad things:
1) OpenBSD is not as fast as the other BSDs. As a matter of fact it is by far the slowest, but practically that is only importand if you intend to reach the system to it's limit (serve under super extreme load etc). On the contrary to what some people claim, OpenBSD, if set up correctly, will not be any slower in a humanly noticeable amount, under normal and high loads, which is the reasonable load that all but the most enormous (let's say 5-10%) of the servers will have. Still, OpenBSD will be noticeably slower on desktop use, in cpu-intensive applications.
2) Many 3rd party applications do not compile or work at all under OpenBSD. OpenBSD's development is quite strict and careful in the implementation of new features. In example, it has been decided that no POSIX capabilities (such as threads) will be included. This breaks compatibility with many applications that rely on such features and work fine in other unix-like operating systems and it a reason for which OpenBSD has the smallest ports collection (about 3.000 applications).
3) Even of those applications that are officially supported (through ports or precompiled packages), most are rarelly at their most recent versions. This is for many reasons, but mainly because the OpenBSD development team is small and has better things to do than babysit 3rd party software, especially since they want to keep OpenBSD secure.
Well that's about all I can think of right now. It may seem I am biased towards OpenBSD (because I like it's development consistency and attitude) but in fact my personal preference is towards FreeBSD.
There are some other BSD-derivative OS, of them I remember Darwin / OpenDarwin and Solaris(SunOS). You can forget about darwin (it's MacOS's core ported to x86 also, but it's not properly supported for anything but an experimental system). SunOS is a very interesting and unique OS and has features than none of the *BSD have -at least builtin-, such as the Access Controll Lists -something similar to SELinux, developed by the NSA-.
About DragonflyBSD it is still far from mature, it's developers are said to plan some wonderful changes (though I wonder if they are wonderful enough enough to justify it's seperation from FreeBSD). Anyway, right now it is not recommended for a production enviroment..
