Apache is the big chief in the world of web servers
The Apache HTTP web server is free to download and is also bundled with many commercial products such as IBM's Websphere and the Oracle database.
Although its strong lead in the web server market has shown some sign of flagging over the past 12 months, it remains dominant, as it has since its introduction in 1995. Netcraft's December 2006 web survey gave Apache just over 60% of the market: double Microsoft IIS's share.
Hundreds of modules have been developed for it, many million websites depend upon it, and so many applications have been designed around it that it is the obvious choice for anyone considering extending their skills in this direction.
And since Apache is an open source product, if you are determined enough, you will find free tutorial material and support to get you going - in marked contrast to the substantial four-figure investment needed to get IIS skills established and recognised. On the other hand, if you would rather pay for commercial training and support, you can get that too.