nginx [Engine X] became the world 4th most popular web server, according to the latest Netcraft survey.
It is an event based web server, which makes really scalable especially for static files, so if you are serving many images, audio files, video, you might consider it.
It can also configured to work as HTTP and mail proxy.
Of course you should look also at Lighttpd [Lighty] which work with the same principle, and I have been using it for around 2 years for alriyadh.com

• apache
• lighttpd
• nginx
• web server