Online statistics for what users are using

For Flash Player: Visit Adobe Site for stats

For OS’s, browser, screen size, colour depth and Javascript: Visit W3C Site for stats

It becomes obvious why publishing in Flash Player 7 is a good idea and testing sites to make sure they work on both IE and Mozilla-based(Firefox) browsers is another good idea. And why maybe avoiding trying to get a site to work perfectly in all browsers (Opera), is not the best investment of time. There are advantages in doing so, but that is mainly if your site is targeting Europe.