NetBSD focuses very heavily on portability. They target lots of architectures, focus on getting cross compilation working well, and spend a lot of time worrying about doing things "right" rather than expediently (for what it's worth; this can be good or bad depending on how you look at it). Their package manager/ports system is known as "pkgsrc", and also focuses on portability, such that it also runs on other platforms like Linux, Mac OS X, and so on. If you want to try porting a BSD to your odd little MIPS-based microcontroller, NetBSD is your best bet.
OpenBSD is focused extremely heavily on security. If you are paranoid, you want to run OpenBSD. If you are not paranoid, you should be. They are out to get you. Don't let them.
And the other major BSDs that you didn't ask about:
Dragonfly BSD is a sort of next-gen fork of FreeBSD, a bit more experimental. The biggest new feature is its HAMMER filesystem, which is in the class of modern filesystems like Btrfs and ZFS, supporting things like snapshots, checksumming of data and metadata, mirroring to backups, data deduplication, and the like. It also has other features like user-space virtual kernels, better parallelism, etc.
PC-BSD is designed to be a user-friendly, desktop-oriented BSD, which manages applications as separate bundles with installers, more similar to how they work on Windows on Mac OS X, than a traditional BSD ports system or Linux package manager which tries to manage shared dependencies (a PC-BSD package comes with all of its dependencies, which are installed separately from those of other packages).
OS X is a hybrid operating system, based on combining the Mach microkernel, FreeBSD, and some NetBSD tools, along with a proprietary UI layer and applications. It runs on desktops, laptops, and ARM-based smartphones. It is well known for its user-friendliness, tight integration between software and hardware, and draconian application development policies for its package management system, the App Store. Luckily, there are alternative ports systems available, such as MacPorts, Homebrew, and Cydia.
NetBSD = run on a toaster
OpenBSD = "Only two remote holes in the default install, in a heck of a long time!"
Exactly what the difference is between DragonFlyBSD and other BSDs I will leave for someone else to summarise in a more definitive way, however my take is that it is all-round more "modern".
It's all rather nicely explained on the front page of their website.
FreeBSD is, in some vague sense, the continuation of the 386BSD project. It's a fork, but a fork in which the parent died out.
OpenBSD is Theo de Raadt's fork of NetBSD. Theo was one of the NetBSD founders and had a falling out with the NetBSD core team.
The origins of all three projects are basically accidents of fate, but FreeBSD has taken up a role as the "optimized for x86/x64 servers" BSD, OpenBSD (obviously) as the "secure" BSD, and NetBSD is the "portable" BSD. In practice, NetBSD sees some use in embedded/appliance scenarios, although Linux has pretty much taken over the world there.
How much NetBSD-originated code remains in Darwin and OS X?
I remember that in the initial betas, there was heavy usage of NetBSD's userland (at least); I've no idea how much remains today.
EDIT: Everyone's mentioning FreeBSD, but I swear to $GOD that I remember seeing 'NetBSD' at the top of many early man pages. NetBSD's site seems to remember something similar: http://www.netbsd.org/gallery/products.html#darwin
A lot of the BSD that's in OSX actually predates either FreeBSD or NetBSD, coming via NeXTStep from BSD 4.3 and via Mac OS X Server from 4.4. In addition there are FreeBSD influences. NetBSD seems largely independent.
That's also roughly what I remember (ex-NeXTie).
Looking at it more closely, it does look like a connection, but I think it is a fairly minor one. The first Mac OS X Server was still pretty close to NeXTStep/Rhapsody.
The bigger changes came later with Mac OS X, which was a major rewrite.
Are we talking the "part of OSX" that is called "Darwin"? If so, the code is all online. Easiest way to determine how much code is from NetBSD is to read it. And diff it against NetBSD's CVS repo.
Is that really just before 2002; or does it include after 2002?
And is there any reason for a big company to donate money / hardware / code / expertise but to ask not to be listed?
A more interesting question is how useful is OSX if we stripped away Darwin? That is, if we just left the proprietary bits, like the graphics code.
Assuming I don't need Apple's graphics, I'd just as soon use NetBSD than Darwin.
Although I don't use ABC BSD, read their code or particpate in making contributions, here's how ABC BSD works ... and here's the lowdown on their code ...
Oh, and the history of ABC BSD is .... I know because I've always been a user of Clickity Click OS and I've been watching ABC BSD.
Take it from me, I speak with authority.
> The simplest way to donate is to sent money via Google Checkout or PayPal.
Should be "send money".