VisBug
Tool for investigating webpages. Especially useful for designers and developers. It has handful of features like measurement, color eye drop or font check. Allows you to change elements dimensions, colors, margins, padding and position.
CSS Pepper
Another tool to investigate webpage CSS. Great for exporting assets, colors, fonts.
Amino
Live CSS Editor. Allows to test CSS/ SCSS in the browser and see results in real time. Also stores CSS you tests in the cloud and works across the tabs.