The free extension Fokus highlights selected text and darkens the rest of your Chrome or Firefox browser window. This makes it a great tool to block out distractions online.
The extension is simple to use. In Chrome, once it's installed, you select the text you want to focus on. It will highlight that in a spotlight and fade out the rest of the page in a dark frame. You can set the opacity of the darkened background, the padding of the highlighted part, and even include a key modifier if you don't want it to be always activated. In Firefox, you will have to hit the "F" icon in the toolbar and then highlight the text.