lighttpd is a small, fast web server with a quite impressive feature set and the clear goal of being faster and more resource-efficient than Apache. CGI, FastCGI, and PHP (via FastCGI) are also supported, making it suitable for dynamic pages as well. Maybe I should take a closer look at it.
leahhttpd is another small web server with a focus on low resource usage and high performance. Here too, there's quite an impressive feature spectrum.
boa is the grandfather of web servers with a performance and resource focus. However, it only offers CGI as an option for dynamic content. So it's better suited for serving purely static content.
Of all three, lighttpd looks the most interesting, among other things because of its good support for interfaces for dynamic content. Especially since the server already has a built-in FastCGI load balancer, making it designed for larger loads right out of the box. And the focus on FastCGI instead of built-in modules offers additional possibilities for security - the FastCGI process can run under a different (restricted) user.