Apache is the most popular web server in the world. It is designed to provide a large number of features and scale across large and powerful servers, though it can even be run on a reasonable home desktop. Apache is available for Windows, Mac OS X, Linux, and FreeBSD.
Lightppd is a fast and lightweight web server. Many sites, such as Wikipedia and YouTube, use it to serve files to users. It supports most of the features of Apache, but consumes less than half as much RAM. Lighttpd is available for Windows, mac OS X, Linux, and FreeBSD.
IIS is a web server designed and developed by Microsoft, and is the second-most popular web server on the internet in terms of usage. It is designed to integrate with proprietary Windows features like ASP and .NET. IIS is only available for Windows 2000, Windows XP Professional, Windows Server 2003, Windows Vista, and Windows Server 2008. It is not included with XP Home, but can be installed through an unofficial workaround.