Internet Explorer (formerly Microsoft Internet Explorer and Windows Internet Explorer, commonly abbreviated IE or MSIE) is a series of graphical web browsers developed by Microsoft and included as part of the Microsoft Windows line of operating systems, starting in 1995. It was first released as part of the add-on package Plus! for Windows 95 that year. Later versions were available as free downloads, or in service packs, and included in the OEM service releases of Windows 95 and later versions of Windows. Internet Explorer is one of the most widely used web browsers, attaining a peak of about 95% usage share during 2002 and 2003. Its usage share has since declined with the launch of Safari (2003), Firefox (2004), and Google Chrome (2008), each of which now have significant market share. Estimates for Internet Explorer's overall market share range from 16.7% to 32.31%, as of May 2012 (browser market share is notoriously difficult to calculate).