Windows 2000 and NT are popular choices for Web server OS's. They offer a familiar interface for IT managers and can be easier to install and set up than Unix Web servers. You should be very careful when using servers on this operating system. Some of the more popular servers are also prime targets for hackers. Servers on Windows are also often not as scalable for serving lots of page views.
Apache is a free, fully configurable Web server available for Unix, Linux, Windows, Mac OS and other operating systems. It is one of the world's most popular Web servers and has many people working on improving it all the time. This server can be very difficult for the novice to install, but there are now companies that offer support plans and assistance to keep your sever up, running, and secure.
Zeus Web server is a highly scalable and secure Web server. It is recommended by people such as the Gartner group as a viable replacement for other less-secure Web servers. It can be a lot more expensive than Apache, but for the price, you get a lot of service and functionality.
iPlanet Web server is the latest offering from the people who brought you Netscape Web server. It is secure and offers flexibility such as Java Servlet capabilties and more. It is a very popular Web server for enterprises.
Windows Server is one of the most popular Web servers available for Windows. It is a powerful server with lots of applications and functions. You have certain applications such as ASP built-in the server. If you use this server, you need to be very careful to stay up-to-date on patches and security. Hackers like to target this server, which can make your site vulnerable if it is not up-to-date.
It is very difficult to find a server that is free and so easy to use and install. The Sambar Server supports a wide variety of applications and if you want more functionality, you can program your own APIs in C or C++. The server is reliable, not, perhaps as reliable as the Unix/Apache Web system, but certainly reliable enough for the average small to mid-sized business.