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This lists sources of browser statistics which have been observed to be regularly updated:
analytics.usa.gov: stats for many visitors to US government websites.
StatCounter: monthly stats for sites using its stats service.
TX3 Stats: stats for sites using Russian TX3 banner-ad servers. Its stats apply best to East European users. It reports an unbelievably large number of Netscape users, so it would appear to be mistaking other user agents for Netscape.
Umeå University: stats for the computer science department in Sweden’s Umeå University.
W3Counter: stats for sites using its hit counter. It reports only the top 10 browser versions: this excludes, for example, 9% of users from its July 2010 report.
W3Schools: historical stats for its own site. This site attracts a special audience, so its stats may not apply to the general population.
WebReference.com: stats sources.
Wikipedia: browser stats from many sources, plus a good discussion of how and why the stats may err.
NB: credible stats are hard to find, especially stats for current sites. Many stats are old, or incorporate old data. If you know of good free stats which cover a recent period of time and which are regularly updated, please eMail me.