Ever since Netcraft has started tracking statistics regarding Web server usage, IIS has never beat Apache when it comes to the number of sites using the two servers. In general, the gap between IIS and Apache has been anything but small. Until fairly recently, early 2002 saw Apache's worst day with only a 30% gap between the two products. Today (in mid-2007), however, Apache's share (52.65%) is very slowly eroding in favor of IIS (32.8%), which is bundled with the ubiquitous Windows OS. Of course, these statistics take into consideration all of the free and cheap hosting services that use Apache and don't consider internal use of IIS in many companies, so the "real" values are likely more similar than they appear.
Even so, 20 percent is still a fairly significant gap! There are a number of reasons that your company might consider making the jump from an IIS solution to Apache. I'll quickly explain some of these reasons and then go over some of the ramifications of such a decision and provide some advice for mitigating problems related to this kind of a move.
