Linux, Apache, Mysql,PHP have been readily available components when making web applications. It carries no licensing restrictions on the number of users and you can swap in any type of database you require, ( However Mysql being the most popular choice).
This make it extremely cost effective to build a web-server farm incorporating a load balanced fault tolerant environment. Scaling in the same manner has not always has been as affordable to achieve.
Windows Web server 2008 apparently will no longer carry some of the previous restrictions such as number of users, type of database used.
This only applies to the Windows Web server addition and then even under certain circumstances.