The venerable Apache HTTP Server is probably one of the most widely deployed pieces of Free Software out there. During its lifetime it has evolved in many ways. It's capable of serving static content, acting both as a forward and reverse proxy, proxy to Java servlet containers etc. Many users are using its integration with various scripting languages to provide web applications (the LAMP stack being the most popular and widespread on the Internet side, and even in many corporate internal networks).
This flexibility does come with a price, though. Apache HTTP server tends to consume a bit more resources than some of its lighter competitors, and its configuration is also not for those of a feint heart.