14% say http://www.w3schools.com/browsers/browsers_stats.asp
(based on a web techie user base)

17% says Mr Nielsen - http://www.useit.com/alertbox/screen_resolution.html

5% says Thomas Baekdal - http://www.baekdal.com/reports/actual-browser-sizes/abs-display/
(based on users of fashion sites mainly)