A couple of sites that document “browser usage” statistics:

http://www.w3schools.com/browsers/browsers_stats.asp

http://marketshare.hitslink.com/report.aspx?qprid=3
http://marketshare.hitslink.com/report.aspx?qprid=6

Wondering if you have any sites that you prefer to use for these
statistics?

This type of data can easily be skewed based on the “types” of
visitors for included sites. I don’t typically have this level of
logging turned on for my sites. However when the question gets asked
or I am interested, I tend to trust a “snapshot” of logging a
particular site to better understand the browsers that visit a site.