According to the statistics on their site, the Firefox web browser has hit the 19 million downloads mark, and will have passed 20 million by the end of this month. That likely makes it one of the most popular Open Source programs ever. There are a ton of people out there that will tell you that Open Source cannot produce applications with the quality of their proprietary cousins. Firefox is an excellent example that not only is it possible to make excellent Open Source apps, more often then not, they end up surpassing their proprietary competitors. (Linux, Apache, OpenOffice and MySQL are other well known examples of well accepted Open Source projects.)
Update:
According to Websidestory, Firefox has now over 5% of the browser market, and has just about pushed IE6 below 90% for the first time in years, not bad for a web browser that was officially released last November. (less then 3 months ago at the time of writing.) 5% doesn’t sound like that much, but if you consider that there are more then a couple of hundred million users on the Internet, 5% is quiet a significant number. (Also keep in mind that Tech people are leading the charge for Firefox, and we don’t download it again each time we give it to someone, so that 20 million number is probably a conservative estimate.)