HTTrack is an easy-to-use offline browser utility. It allows you to download a Web site from the Internet to a local directory, building recursively all directories, getting HTML, images, and other files from the server to your computer. HTTrack arranges the original site's relative link-structure. Simply open a page of the mirrored Web site in your browser, and you can browse the site from link to link, as if you were viewing it online. HTTrack can also update an existing mirrored site, and resume interrupted downloads. WebHTTrack is a Web-based GUI for HTTrack.
SWEC is a program that automates testing of dynamic Web sites. It parses each HTML file it finds for links, and if those links are within the site specified, it will check that page as well. In addition to parsing and locating links, it will also parse the pages looking for known errors and report those. It will report if a page cannot be read (by either returning a 404, 500, or similar).
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.