Firefox 1.0, 2004
Internet Explorer pwned the browser world through the dawn of the new millennium and beyond, yet satisfaction with the product was far from universal. Some felt Explorer was unnecessarily ponderous. Others were concerned over security issues or the way it was "forced" upon users as part of the Windows package.
It wasn't forced, of course, but when a promising open-source challenger appeared in 2004, the disillusioned had a viable place to turn. Firefox 1.0, with its roots in the remnants of Navigator and built by Netscape spin-off Mozilla Foundation, was far from perfect, but time has established it as a key turning point. Going forward, Firefox would stick and the browser biz has been a far more competitive place ever since.