So, Sun has finally gone open source with its OpenSolaris OS. Sun Chairman and CEO Scott McNealy said Sun was the largest donor of code anywhere on the planet. "You can use (Solaris) free. Did I say free? I meant free," said McNealy.

But what does open source, from Sun's perspective, mean? And how does Sun's model compete with the GPL-based model that's used by Linux -- the one that encapsulates what most people understand by open source?

The reason this is important is clearly that, if you want to use the open source advantage to the full, and combine source code to build software that does exactly what you want it to do, then both will need, ideally at least, to operate under the same legal framework. The alternative could be a mess that looks like SCO. Sadly though, you can't do that.

In one corner is the GPL, or General Purpose License (sic), a free licence. It allows you to use the software in pretty much any way you see fit. One key proviso though is that any code that becomes part of the distributed product must also be published under the GPL. It's a way of ensuring that there are no legal arguments further down the line about which bit of code belongs to whom. It keeps the cash situation simple.

Sun's software on the other hand runs under the company's CDDL (Common Development and Distribution License). Open source founders at the GNU Project have this to say about Sun's licence:

"This is a free software license which is not a strong copyleft; it has some complex restrictions that make it incompatible with the GNU GPL. That is, a module covered by the GPL and a module covered by the CDDL cannot legally be linked together. We urge you not to use the CDDL for this reason. Also unfortunate in the CDDL is its use of the term 'intellectual property'."

Sun points out that it bases its licence terms on the MPL (Mozilla Public License), of which GNU says: "A module covered by the GPL and a module covered by the MPL cannot legally be linked together. We urge you not to use the MPL for this reason."

The GNU Project states though that there's a let-out clause in the MPL which allows a program to offer a choice of another licence, in which case that chunk of the program can be considered GPL-compatible.

While Sun's CDDL has retained that part of the licence agreement, other areas of the CDDL make it a very different beast from the GPL. What it boils down to is that the two models are inherently incompatible because software distributed under one or other of the two licensing schemes cannot be inter-mixed.

What makes the situation interesting though is that the Linux movement, which dominates the open source field now has a big competitor in the shape of Sun. True, it's only a single adversary, and there are by comparison thousands of developers, large and small, beavering away creating Linux code. And OpenSolaris doesn't have the economic might of IBM, Red Hat or Novell propelling it along -- although to Sun's credit it did turn a profit in its last quarter a couple of weeks ago. At $19 million, it's not much for a company with revenues of $2.843 billion over the same period, but it's the first for some years. It means Sun will increasingly become an economic force to be reckoned with.

So, if you want open source software -- or at least an alternative to Microsoft, you can select OpenSolaris or Linux. They're different in multiple ways, from technology to business model and licensing. Whether one is better than the other depends on all the usual factors that are not within the power of those outside your organisation fully to determine.

But you do now have a choice.