Highly is a free web highlighter for Chrome.

Installing the extension adds a button to your address bar, and clicking this turns your mouse cursor into a highlighter pen: just click and drag across your selected text to highlight it.

Saved highlights are visible if you revisit the page later, and can be searched, browsed or shared on Twitter, Facebook, Pinterest or by email.

Highly can also be directly used as a social network, with options to share your highlights publicly, and follow others or have them follow you (here's an example).