If I'm not mistaken (please correct me if I am), a plurality (if not an outright majority) of web servers are run using an open source OS. jQuery/Prototype/MooTools together hold an obscenely high share of the JavaScript library market. The great majority of websites on the internet run some combination of PHP, Python, Ruby, many of those running open-source frameworks like Pylons, RoR, CodeIgniter, Kohana, web2py, Django, etc. A great number of developers code using Vim, Eclipse, or some other open source text editor.
And from the perspective of the user? It's hard to quantify this just by saying "Firefox" or "Chrome" or the "V8 JavaScript Engine". I think open source projects (like jQuery) benefit hugely from crowd sourcing their problems. By making their bug reports and code public, they allow someone like me to fork the project on GitHub, fix the bugs, and issue a pull request back to the master branch. They're basically fine-tuning their product without doing nearly as much work as closed-source shops without paying the person for fixing the problem. To the user, it looks like things get done quickly and bugs tend to come far less often (if the crowd sourcing is done right).
In my humble estimation, these are rather important considerations when judging the success of open source...and these are only web technologies.