According to an October survey by Netcraft, Apache, the world's most dominant web serving software (which happens to be free), surpassed 50 Million websites
in total, based on around 75 million surveyed. That gives Apache (which apparently includes apache and apache2) around a 70% market share in the world today. Coming in second was Microsoft's IIS with 15 million, and various others, which would include Sun and Google's custom web servers, suites like OmniHTTPD and all the rest filling the bottom 5%. There isn't any OS data to go along with these numbers, but it makes sense that Apache would have such a large share because it is released for multiple platforms and it usually out of the box support for most server software.