Chapter 2: Improving Page Load Performance: Pingdom, YSlow and GTmetrix
by Tonino Jankov
GTmetrix is a rather advanced tool that offers a lot on its free tier, but it also offers premium tiers. If you sign up, you can compare multiple websites, multiple versions of the same website, tested under different conditions, and save tests for later viewing.
YSlow is still relevant, although its best days were those when Firebug ruled supreme among the browser inspectors. It offers a Chrome app and other implementations — such as add-ons for Safari and Opera, a bookmarklet, an extension for PhantomJS, ...