Whose Linux are you using? (I find this particular penguin, from Penguin Computing, to be cute.)
That is, which distro do you or your company depend upon?
Estimating the "market share" (more appropriately mindshare) of various Linux distributions is difficult and, some say, fairly useless.
The best way to do this is through a survey. Those who choose to answer are counted, we assume people are answering truthfully.
Two recent surveys show markedly different results.
Evans Data released a survey of developers in April. It indicates non-commercial, free versions of Linux are gaining traction. John F. Andrews, COO of Evans Data, said in a press release that the cost and ease of updates were becoming more important than the support claimed by outfits like Red Hat.
A user survey from Desktop Linux, meanwhile, painted a markedly different picture. It found a very tight race among Mandrake, Debian, RedHat, SUSE and Gentoo, all with around 15% of the desktop market. The survey concluded its readers were quick to switch, meaning no lead is ever a safe one.
DistroWatch, meanwhile, just counts traffic to the home pages of various distro offerings. They have Ubuntu in the lead, followed by Mandrake, Fedora and MEPIS.
The key question is, does this matter? You run the distro that best meets your needs. You may care most about support, about updates, about new features, or about a specific feature set, like local language support. The learning curve you face in moving among distros is not great, and the cost of change will vary depending on the size of your installation.
What I'm interested in, right now, is learning more about what folks who run Linux want in a distribution, what they think may be lacking in the software they use, and what they would like to see improved. I expect answers here to be all over the map, because everyone is different. But it's a good place to start after a long holiday weekend.