First shameless self plug for whomever watches YouTube videos, Twitch streams, or consumes instagram via the web (twitter soon too) our Social Blade extension brings stats directly to the video/image page: https://chrome.google.com/webstore/detail/social-blade/cfidk...

Beyond that here are a few others I like that I haven't seen mentioned:

hckr news: https://chrome.google.com/webstore/detail/hckr-news/mnlaodle... (marks what new comments are here)

Honey: https://chrome.google.com/webstore/detail/honey/bmnlcjabgnpn... (shows price charts on amazon products plus finds a lower priced one if amazon shows a higher priced one first)

Momentum: https://chrome.google.com/webstore/detail/momentum/laookkfkn... (inspiring images on new tab page)

Pushbullet: https://chrome.google.com/webstore/detail/pushbullet/chlffgp... (get my android notifications on my desktop/laptop)

Twitter Account Switcher: https://chrome.google.com/webstore/detail/twitcher-twitter-a... (for changing between more then one twitter account)

Unix Timestamp Converter: https://chrome.google.com/webstore/detail/unix-timestamp-con...

Gestures for Google Chrome: https://chrome.google.com/webstore/detail/gestures-for-googl... (can close tabs, switch tabs, etc with right clicking and dragging mouse etc)