Really detailed tool that diagnoses in great detail each step of the DNS resolution process for a domain. Free and invaluable.
Mobile performance testing platform from akamai based on the webpagetest.org infrastructure, free. Useful.
Performance testing, monitoring, and some kind of fix-it service, mostly in private beta. Flashy, funky, cool, style design and chat with limited technical details.
Very handy tool that allows one to preview a site as it might look in a variety of browsers on a variety of platforms. Clever.
Link from Dafydd. Interesting business offering cost per acquisition based online marketing services. Worth pursuing at the relevant time.
Web browser testing and automation framework / suite. Similar to selenium. Cross platform, cross browser. Hat tip sfllaw.
Selenium is a test tool for web applications. Selenium tests run directly in a browser, just like real users do. It runs in Internet Explorer, Mozilla and Firefox on Windows, Linux, and Macintosh, Safari on the Mac.
twill is a simple language that allows users to browse the Web from a command-line interface. With twill, you can navigate through Web sites that use forms, cookies, and most standard Web features.