Windows Internet Explorer (formerly Microsoft Internet Explorer abbreviated MSIE), commonly abbreviated to IE, 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 has been the most widely used web browser since 1999, attaining a peak of about 95% usage share during 2002 and 2003 and steadily declining since. After the first release for Windows 95, additional versions of Internet Explorer were developed for other operating systems: Internet Explorer for Mac and Internet Explorer for UNIX (the latter for use through the X Window System on Solaris and HP-UX), and versions for older versions of Windows. Only the Windows version remains in active development; the Mac OS X and UNIX version are no longer supported. Internet Explorer was first released as part of the add-on package Plus! for Windows 95. Later versions are available as free downloads and are also included in the OEM service releases of Windows 95 and in later versions of Windows. The most recent release is version 7.0, which is available as a free update for Windows XP with Service Pack 2, and Windows Server 2003 with Service Pack 1 or later, and is included with Windows Vista. An embedded OEM version called Internet Explorer for Windows CE (IE CE) is also available for WinCE based platforms and is currently based on IE6.