Web designers and marketers alike design websites that must reach the largest audience as possible. The message and presentation must attract that audience so they will be compelled to take action. Web browsers can cause havoc during the design phase. Here are resources on web browser usage as of March 2011.

"The usage share of web browsers is the percentage of visitors to a group of websites that use a particular web browser. For example, when it is said that a web browser has 40% usage share, it means that some version of that web browser is used by 40% of visitors that visit a given set of sites."

World wide usage share of browsers for January 2011

Source	I.E.	Firefox	Chrome	Safari	Opera	Mobile
Net Apps	56.0%	22.8%	10.7%	6.3%	2.3%	3.8%
StatCounter	46.0%	30.7%	15.7%	5.1%	2.0%	4.3%
W3Counter	40.0%	31.1%	14.2%	5.9%	2.0%
Wikimedia	41.6%	28.7%	11.8%	9.3%	4.3%	6.9%
Median	43.8%	29.7%	13.0%	6.1%	2.2%	4.3%

Wikipedia

Generally, Internet Explorer seems to be losing ground while Firefox, Opera, and Safari stay fairly steady, and Chrome and mobile platforms gain ground.

IE6 Finally Nearing Extinction

"For years, IE6 caused headaches for developers and prompted many users to switch to alternative browsers. It was full of security holes and it broke nearly every web standard in the book...

Now, based on an analysis of 15 billion page views in May 2010, StatCounter’s numbers indicate that IE6 usage in the U.S. has fallen to 4.7% from 11.5% in the last 12 months, meaning that IE6 is finally becoming a footnote in the history of the Internet."

Mashable

The IE6 trends is a welcome trend for most designers. Supporting an ancient (by tech standards) platform is cumbersome and challenging. Many groups today have simply chosen not to support it any longer. It really depends who the primary audience is. If the audience is highly unlikely to even use IE6, there's no reason to support it.