I like this extension. It looks just like the images (only in my theme's colors and in English). I like that I have two options for inputting calculations (clicking on the buttons and typing them with the keyboard). I've had no issues with it.

Something to note: When you google to the chrome webstore don't login with you google account or it will still say add to chrome, however, if you're not logged in it will say add to opera but only when accessing the site from an opera browser.