Google is leading the pack for browser vendors, thanks to Chrome's built-in update feature. 90.6 percent of all Chrome users have the latest stable release (version 7). Furthermore, considering Chrome's fast release cycle, it's very likely that many of the users not on version 7.0 are actually using a later build (either version 8 or version 9). Following Chrome, the next three are Firefox (81 percent are on version 3.6), Opera (77 percent use version 10.6), and Safari (71 percent run version 5.0).
It's not very surprising that Internet Explorer is dead last. Many haven't upgraded to the latest version of Microsoft's browser, though it's still impressive to see that 60.2 percent of IE users are running version 8. IE8 is actually the most popular browser, even if you don't take version numbers into account for other browsers. Furthermore, many are eagerly giving Internet Explorer 9 a shot.
Now the question is, are you running the latest stable or beta version of your browser?