mod_sphinx is an Apache module which intercepts output bound for the client, whether it be from files served by the same Web server, files sent via reverse proxying, or files delivered by transparent forward proxying (outbound proxying, as in your browser's proxy setting). It opens the text and replaces a word with another word or phrase. When in regular proxy mode, it cannot modify SSL content since Apache cannot decrypt that (it would be a protocol violation).
Hammerhead is a stress testing tool for Web sites. It initiates connections from multiple IP aliases and simulates a user from each alias. It is fully configurable, and there are numerous other options for creating problems with a site. Extensive data collection is also available.