Internet Explorer 9 has been in beta for a long time. Then came the release candidate. While this doesn't compare to how long Firefox 4 has remained in beta, Microsoft has finally released a stable version of IE9.I've been using it for a long time now (never made it my default), and it looks really promising. Performance is a lot better compared to previous versions, and Internet Explorer has finally changed considerably.
If you have been using the release candidate, a download should come automatically through Windows Update. If not, you can manually download the browser, and stay up to date, right from Microsoft.
