Firefox Developer Tools - Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.
HttpWatch - HttpWatch: The ultimate HTTP Sniffer for Chrome, IE and iOS
Google Chrome Developer Tools - Get started with Google Chrome's built-in web developer tools.
Website Speed Test by Pingdom - Pingdom is a website performance monitoring software.
TrueMetrics Analytics Inspector - Analytics Inspector provides an easy way to validate web analytics and marketing integration, as well as offers a great way to quickly validate the instrumentation via the unique and simplified user interface.
Yellow Lab Tools - Website test that checks performance and HTML, CSS and JavaScript quality.