I consistently find myself looking up statistics for OS market share around this time every year. Every year numbers change. Every website reports different numbers from the last, and the only thing that one can assuredly deduce is that it is impossible to accurately gauge how many systems are running a specific OS. With that said, market share does matter.
For software engineers, time is very limited. It takes much of one's limited resources to make quality software, and it takes quite a bit of time. This means that choosing target platforms is very important if you plan on being successful with a certain product. It would be folly to write server software for a system that will not succeed in the server market. Likewise, it would be folly to write entertainment software for a system that will never succeed in the entertainment world. Strangely, Linux is only a large force in mainframes and super computers. There Linux is not only a contender, but is instead the only contender. In the server realm, we can reliably say the until rather recently Linux and BSD were the strongest. Microsoft has caught up quickly and can now claim roughly 40% as can Linux, and everyone else makes up the remaining 20%.
What's interesting is the hold that Microsoft has on the desktop market. According to the w3counter: OSX is 7.83%, Linux is 2.78%, iPhone, WAP, and Android together make up about 1%, and everything else is Windows. What's funny is that Apple is now the 7th largest computer manufacturer in the world. They're also the only one to have beaten the industry average in growth for Q1 of 2010. What does this mean?
This means that there is still a chance for the rest of us. Apple has proven that Microsoft's empire is not invulnerable. If people can recognize the differences between Mac and PC, they can recognize the difference among Linux, Mac, and PC. With that said, I think that people are also learning the reality. Windows just sucks. I am not saying this from a fanboy stand point. I dislike Macintosh too. I don't even really care for Linux. I simply think that Linux and OSX are better products than Windows (security, stability, ease of use, performance, scalability, total cost of ownership). The key to this is simply Total Cost of Ownership.
With a Windows system, people will suffer from many different things. First, the cost of upgrades: software (Windows and applications for Windows will need to be purchased), labor for IT personnel, and hardware upgrades/replacements. Second, the cost of maintaining Windows machines: anti-malware/security software (usually annually recurring costs), IT personnel to monitor system health, compatibility management with Windows update, and security staff that are well aware of social engineering attacks.
With Macintosh, you have a higher initial equipment investment, but afterwards you need only worry about social engineering attacks (usually, this is unauthorized physical access to a certain machine). Upgrade costs are minimal (around 30USD for each OS upgrade), and backwards compatibility among OSX versions is fairly acceptable.
With Linux systems, you set it and forget it. The hardware cost isn't high. The only cost here is protection from social engineering attacks (like Macintosh, these are usually unauthorized physical access to a given machine). So far, all three are susceptible to tampering when in person. You simply need a liveCD/USB, and/or a laptop, a screwdriver, and an ATA-USB bridge... and all the data can be yours if you are so inclined. This will never change, but what is changing is important. Linux, OSX, BSD, and Solaris are all far more secure than is Windows. This is not due to obscurity. Linux is hardly obscure. It competes with Microsoft in the most valuable market, servers. Do I want one credit card number, or one million credit card numbers. I will go with the million. Windows suffers because it's an easy target. The differences are largely architectural. OSX, BSD, and Solaris share this architecture and enjoy its benefits.
So where does market share difference leave us? The same place we started, except we can look forward to a better future. The trend lines show Unix-like systems winning the OS wars. Microsoft is still the giant, but Linux and OSX are gaining.