All of these are free and available in the Chrome Web Store.
1-click timer: Really simple timer app.
Clipboard History 2: Makes sure that you never lose the content on your clipboard by accidentally copying something else.
Cloudy Calculator: Simple but very powerful calculator.
Dark Reader: Turns all white on web pages to black and vice versa while leaving most color alone.
Data Saver (Beta): Reduces bandwidth usage on many sites.
Disconnect: Blocks advertisers/trackers/companies from tracking you; by doing this, it makes webpages load faster.
FlashControl: Blocks flash videos from playing unless you tell them to.
Grammarly: The much-awaited grammar-checker.
HTTPS Everywhere: Forces many websites to provide you with a more secure version of the website that you're on.
Pushbullet: Easily send a webpage or note to your phone in an instant.
StumbleUpon: Discovers your interests and lets you “stumble” to a random web page that you might like.
Tab For A Cause: Gives money to charity for every tab you open.
Wikiwand: Improves the interface of Wikipedia automatically.
WOT: Warns you if you are entering a website with a bad reputation from other WOT users.
Zenmate: Like Surfeasy, but completely free. However, it only works in Chrome.