Chrome Developer Tools is a powerful and often underutilized toolbox. http://code.google.com/chrome/devtools/docs/overview.html

Inspect the DOM and set modification and event listener breakpoints: http://code.google.com/chrome/devtools/docs/elements.html

Assets, Cookies, Databases: http://code.google.com/chrome/devtools/docs/resources.html

Network timings: http://code.google.com/chrome/devtools/docs/network.html

JavaScript debugger, breakpoints, watch expressions: http://code.google.com/chrome/devtools/docs/scripts.html

Profile everything, network, scripts, styles, layout, painting, garbage collection: http://code.google.com/chrome/devtools/docs/timeline.html

JavaScript CPU and Heap profiling: http://code.google.com/chrome/devtools/docs/profiles.html

JavaScript console, see errors, execute code, inspect breakpoint scope: http://code.google.com/chrome/devtools/docs/console.html