Memex : Browser Extension to full-text search your browsing history & bookmarks.
Browser Calories : The easiest way to measure your performance budget.
SlimerJS : A scriptable browser for Web developers
Cypress : JavaScript End to End Testing Framework. Fast, easy and reliable testing for anything that runs in a browser.
Arora : Arora is a lightweight cross-platform web browser. It's free (as in free speech and free beer). Arora runs on Linux, embedded Linux, FreeBSD, Mac OS X...
Measure between the following elements: images, input-fields, buttons, videos, gifs, text, icons. You can measure everything you see in the browser.