There are a variety of platform-specific and generic keyboard shortcuts that you can use to increase your efficiency.

It's possible to send a composed message without clicking the send button by using:

Cmd (Command) + Enter on Mac/Apple or Ctrl (Control) + S on Windows
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Using either of the key combinations will immediately send a chat or email response without the need to use your mouse. While you'll still be able to click the send button as you can today, learning this combination should significantly improve the speed at which you can reply.

New Paragraph

Enter

New Line

Shift + Enter or Ctrl + Enter

Ctrl + Z

Ctrl + Y

Ctrl + C

Ctrl + V

Ctrl + A

Ctrl + N (or T)

Ctrl + F 

This is extremely helpful because it lets you search inside of a web page that doesn't have a search bar. It'll only search the current page, though, and it's looking for an exact match.

In most browsers, you can double click on a string of text to select it. You can then use one of the shortcuts above to copy/paste or perform other actions.

Try it here: Double_Click_To_Select 

In most browsers, you can right-click a URL to open it in a new tab. Some URLs will automatically open in a new tab, but this is a way to force others to do the same.

In most browsers, you can right-click an image to open it in a new tab. This will allow you to view the full-size version of the image.

On a Mac/Apple computer, you'll need to use the Cmd (Command) key instead of Ctrl (Control.) Please note that these keyboard shortcuts won't work in all situations/browsers, and their behavior may vary.