People will always babble on about market share and installed base. Linux Penguins will tell you that the world is moving away from Microsoft and towards the Open Source Revolution. Macintosh people will tell you that the world is moving away from Microsoft and towards the Macintosh, especially now with OSX and the iApps, and so forth.
How can you tell who is telling the truth?
Well, it turns out that there is a very good way. Almost everyone who has bought a computer recently uses it to access the Internet, and anyone who isn't terminally brain-dead knows that Google is the best site, bar none, to do searches. And everyone does searches.
So, here is the data, compiled from this source: http://www.google.com/press/zeitgeist.html Some of the results may surprise you.
An explanation of the numbers....
Figures prior to November 2001 are estimated for both Windows 2000 and Windows NT... prior to this date, Google tracked both as "Windows NT". Only after the release of Windows XP did Google start differentiating between the three NT-based OSes.
What do these figures actually mean?
I should clarify the difference between market share and installed base. Market share is actually defined as the percentage of computers of a certain brand or platform sold divided by the total computers sold in a certain period (a quarter or a year, usually) This is different from Installed Base, which measures the percentage of computers of a certain brand or platform that are STILL IN USE, divided by all computers that are currently in use. Obviously, installed base is a lot more difficult to calculate. The Google numbers actually measure Googling Installed Base and not market share-- this is why Apple can have a Google percentage of 4% yet its most recent market share lies below 2% worldwide.
What is "Other"? Why is it so high (4-5 percent)?
While Google does not give explicit details on the breakdown of the "Other" category (I've asked) you can get pretty good approximation by looking at detailed stats for other high-traffic sites provided by stats packages like awstats.
The vast majority of the "Other" category is comprised of bots and spiders, automatic robots scanning the web to provide data for their search engines. These are not human users at all and can be discounted. The remainder, a tiny tiny minority, is made up of things like the BSDs, Amigas, WebTV, and cellphones (all of which have approximately the same share, about 0.1-0.3%)
Why do the figures only go up to July 2004?
Because some asshat went whining to Google about how they felt the figures had hurt them personally, and some even bigger asshat at Google got all scared and yanked the numbers permanently. Asshats.
Fortunately, however, there is a second, although not as extensive, set of numbers available at www.thecounter.com. This company provides stats for a wide variety of different websites, and monthly unique visitor stats are in the area of 100 million, so these numbers should be nearly as good as Google's were.
And just for fun, here's a chart of web browser installed base, also from TheCounter.com: