If your Web site gets (or you anticipate that it will get) a large amount of page views on a regular basis, then you should investigate Web servers that can handle this load. Servers should be scalable and able to adjust to changing levels of requests. Good servers will also have load-balancing capabilities and offer functionality to continue to serve pages even under extreme conditions.
Part of the reason that Apache is one of the world's most popular Web servers is because of its ability to scale to match the needs of the site it is serving. Being open-source, it has many developers working on it to make it faster. It is also free, which makes it a great choice for mid-sized businesses that don't want to spend a lot of money on a Web server.
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.
Formerly the Sun ONE Web server, the Sun Java System Web server provides a powerful and scalable business Web server solution.