When targeting the internet consumer knowing what they are using for Operating systems and browsers is necessary. This determines what browsers and operating systems you are going to support.
Additionally monitoring usage changes allows you to predict when you need to add or remove a supported browser or operating system.
This can be done with two tools, one google analyics or a report from net market share .
http://marketshare.hitslink.com/operating-system-market-share.aspx?qprid=8
Even though Windows 7 was released in Oct 2009, windows continues loosing market share over the past 6 months. This is the general consumers usage pattern.
For your web site, things might be completely different, a Mac OS site may naturally have mainly Mac users. Where as a game site for a PC based game would mainly have windows. These are obvious factors.
If you are targeting the general user and your profile does not match net market share’s report then finding out why may teach you more about your customers or highlight unknown browser compatibility issues.
Read cross browser testing for an example of the importance of cross browser testing.