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The landscape seems crowded to me and since as I'm not into BSD systems, I thought I'd better ask the question here. "

FreeBSD, NetBSD, OpenBSD, and DragonflyBSD are the most popular "flavors." There are other flavors, but you'll find that these four make up the bulk of *BSD systems.

These flavors are commonly confused with Linux distributions. Unlike Linux distributions, which all use the same kernel and really only differ in the userland tools they provide, each "flavor" of *BSD has its own kernel, own base system, own userland tools, and own package management system. Each *BSD flavor its its own full-featured OS.

All of the current *BSD's are based on the original 4.4-BSD operating system. FreeBSD and NetBSD are direct ports, while OpenBSD and DragonflyBSD are forks of NetBSD and FreeBSD respectively. Since they share the same heritage, they also share a lot of code which makes porting tools easy. This is why you see DragonflyBSD using tools from other flavors and visa versa.

It is far less crowded than the linux variants (and better segmented), and there is far more code reuse.

There are 4 major BSD OSes (listed alphabetically):
* DragonflyBSD
* FreeBSD
* NetBSD
* OpenBSD

There are various permutations to each of these OSes (pfSense, FreeNAS, EchoBSD, PC-BSD, etc). However, these tend to build upon one of the 4 BSDs, with specific things added or removed to make it work in particular niche.

The BSDs tend to co-operate a lot and share code around (PF, wifi, and NIC drivers are great examples).

There are also commercial forks of BSD systems. For example, JunOS, used in Juniper routers, is a fork of FreeBSD.

And MacOS X uses a lot of FreeBSD and NetBSD code.

There's also the original commercial BSD/OS, which is no longer available.

So, while there's more than one BSD OS out there, the environment is nowhere near as crowded as the Linux distro environment.

Thanks to all for the information. The "crowded" qualification was due to ignorance on my end and hasty judgment I must confess. In fact, in the light of foldingstock's comment, the differences are more fundamental compared to Linux distributions yet there are less "variants"/"flavors"...

Is there binary compatibility? Between DragonflyBSD and FreeBSD for instance? Or some sort of "acceptance" in the sense of one system accepting packages built for an ancestor?
I guess these four flavors are desktop OSes? Is the user base significant? At what level should BSD as a whole be placed? The same as Linux? or somewhere between Linux and the likes of AROS, MorphOS, Haiku, etc.?

I wish PureDarwin had taken off. It could be a very good member of the team.

Just a quick note. These ones are sometimes called distributions, because they are BSDs with different software and settings.

While DragonFly, FreeBSD, NetBSD and OpenBSD are usually called derivatives, because they are forks of each other but in reality different operating systems. I'd consider MacOSX a of that family.