Cost can be a significant factor in making applications architecture choices. Red Hat Linux and Apache are free, and that has made them a very attractive choice to smaller businesses, educational institutions, and so on, especially because they offer "openness" as well.
Microsoft gives away IIS with every copy of NT or Windows 2000. However, NT is a comparatively costly operating system.
Of course, costs must be weighed against benefitsit's no use having a free Web server on a free operating system if your client's in-house skills and systems are all Microsoft-biased. Also, Windows 2000/IIS outperforms the Linux/Apache combination for some dynamic e-commerce bench tests; for example, in handling a greater number of requests per second on ZDNet's bench test.