This lists sources of browser statistics which have been observed to be regularly updated:
EPA: stats for the EPA website.
Internet Archaeology: stats for the Internet Archaeology site.
Net Applications’ Market Share: stats for sites using its stats service. Monthly stats, updated monthly. In Aug 2010 the stats became far less useful because they began reporting just the top 5 browser versions; this excludes, for example, 22% of users from its July report.
Pitt County: stats for the Pitt County, North Carolina website. Traditionally these have been monthly stats, updated irregularily, usually once a month, but sometimes less often. Related monthly stats are also available for a second server which appears to serve more users.
StatCounter: stats for sites using its stats service, updated daily, albeit with some lack of detail.
TX3 Stats: stats for sites using Russian TX3 banner-ad servers. Its stats apply best to East European users.
Umeå University: stats for the computer science department in Sweden’s Umeå University.
USGS: stats for the US Geological Survey External Server. Users are mainly from US government and educational institutions. Updated daily, but always a few days behind. Also offers stats for prior months. Note : it sometimes reports major inexplicable surges of IE5 usage.
WebHits: stats for mainly European sites using its hit counter.
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 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: daily browser stats for its own site. This site attracts a special audience, so its stats may not apply to the general population. Daily stats may fluctuate a lot.
Note: Credible stats are hard to find, especially stats for sites which attract ‘typical’ audiences. 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.