A free hosted version of the open source monit-aggregator written by Matt Fawcett. Appears to be free with no node restriction.
Source code of monit-aggregator by Matt Fawcett / runslikeclockwork.com. He offers a free hosted version at that site.
YSlow is an extension for Firebug which shows ways to speed up page loads. It's published by Yahoo as part of their developer network.
The Linux Virtual Server as an advanced load balancing solution can be used to build highly scalable and highly available network services, such as scalable web, cache, mail, ftp, media and VoIP services.
Linux HA produce the Hearbeat application which is used in high availability clusters.
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.
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.
Yahoo!'s Exceptional Performance team evangelizes best practices for improving web performance.