Firefox

It took five years, three major versions and the contribution of thousands of volunteers, but Firefox has reached one billion downloads. Not so much a technical milestone, the number has a greater importance in reminding what it took to get here and how the browser market, and the Internet as a whole, have evolved. As you can imagine, the celebrations at Mozilla are already underway, but there's no time to rest on its laurels, and the team has already launched a new site to help spread the word even more.