iTunes 4 for Windows
Though not deserving of its own numbered release, the release of iTunes 4 for Windows on Oct. 16, 2003 was a monumental strategy move from Apple. With the iTunes Music Store now accessible on Windows-based machines, Apple opened up the iPod market in a huge way. While iTunes on Windows wasn't exactly the smoothest software, it played an integral role in the domination of the iPod worldwide. By the time iTunes for Windows launched, the iTunes Music Store boasted nearly 400,000 individual tracks.