Nginx / lighttpd light weight web server.

Apache is a standard for web server. But, you can help apache and utilize computing resource by using lighttpd and nginx as a helper for static content/load balancer for your website.

nginx :
http://wiki.nginx.org/NginxModules
lighttpd
http://redmine.lighttpd.net/wiki/lighttpd

webserver http-caching header

Here are link to speed up your http server :

http://www.mnot.net/cache_docs/
http://www.web-caching.com/