http://varnish.projects.linpro.no/
Varnish is a state-of-the-art, high-performance HTTP accelerator (reverse proxy). Varnish is designed by a BSD kernel developer to make full use of BSD / Linux kernel's ability to manage memory. It is extremely fast, up to 10 - 20 times faster than squid.
http://www.danga.com/memcached/
memcached is a high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load.
http://developer.yahoo.com/performance/
Yahoo!'s Exceptional Performance team evangelizes best practices for improving web performance.
