According to the latest (June 2009) NetCraft web server survey, the Free/Open Source Apache web server is now hosting 50.46% of all active web sites surveyed (about 38 Million). Microsoft’s IIS server is at 28.05% (or about 29 Million) - a 7.64% decline from IIS’s May 2009 statistics. Interestingly, Google holds 12.2%, presumably most of that is their own content or content generated by their applications.
This is nothing new - both Free/Open Source software and Unix-related stuff has always had a stronger share in the server (and Internet) market than Microsoft products. And, despite all of Microsoft’s FUD, it’s clear that Apache is still more popular than IIS by a large margin - probably in no small part due to the extendability and scalability of Apache, and its security record (just take a look at the difference in system calls).
The real shining example, however, comes from looking at the stats on the Internet’s million busiest sites - 66.26% running Apache and only 18.77% running IIS, which has been constant for the better part of the last year. That says quite a bit about the stability and scalability of Apache. Not to mention that a lot of the really big sites run their own custom-modified versions of Apache which may or may not be identified as Apache in a survey.