This is a little update on Steve‘s suggestion. You can view past updates for OSes in 2008, browsers in 2009, OSes and browsers in 2009, and in 2011. And just as before, we still have a few hits from IE 2.0. Though in what seems to be common trend across a lot of sites out there, Safari and Firefox are far ahead of Internet Explorer. Don’t know if this will change with the proliferation of Windows phones and Safari most likely includes a sizable number of iPhones.
On the OS front, Windows is still ahead with XP having the same usage share of Mac OS X. I believe “Windows NT” includes Windows 7/8 and Server 2008.