Submission + - Market share: MS below 90%, Firefox up to 20%
A CNN blog has a write-up on it that contains some information on how this is measured:
Net Applications' monthly surveys are conducted by sampling browser data from some 160 million visits to Web sites operated by firm's clients. Although the company describes the results as "market shares," Net Applications does not actually measure share of market in the traditional sense of sales revenue or unit sales. It does, however, provide a consistent methodology by which to measure browser and operating system trends.
So this is only a crude metric, but as the blog mentions and in the data linked to above, a decreasing MS "market share" has been showing up consistently over the past few years. Is the MS monopoly starting to erode?