It is important to know what browsers are being used on our sites since you are the ones building them. Here is a list of the top browsers, by percentage. One thing to note, internally we use about 50/50 Chrome and Firefox. With more Firefox 4 users than Firefox 3.x. Compare that with our users statistics.
42.0% - IE 8
12.5% - IE 7
12.0% - Safari(multiple versions)
12.0% - Firefox 3.x
6.5% - Chrome
5.0% - Firefox 4
5.0% - IE 9
5.0% - other (mobile, ie6, etc)
What this shows is we are actually building in some of the least used browsers by our users. This is a reminder that if you are making changes to a site, or creating a new one, IE is still king amongst our users. IE 8 needs to have plenty of time committed to testing your site against. Any changes you make to markup or widgets or styles must be checked in IE 8 to make sure they are working correctly for a significant portion of our user base.