Internet Explorer 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 Original Equipment Manufacturer 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 Firefox and Google Chrome, and with the growing popularity of operating systems such as OS X, Linux, iOS and Android that do not run Internet Explorer. Estimates for Internet Explorer's overall market share range from 27.4% to 54.13%, as of October 2012. Microsoft spent over US$100 million per year on Internet Explorer in the late 1990s, with over 1000 people working on it by 1999.