Open your application switcher
Command + Tab
From there, you can navigate with the arrow keys.
Command + Shift + Option + Esc for three seconds
This bypasses that annoying dialogue box that asks you if you're sure every time.
Screenshot your entire screen
Command + Shift + 3
Not just a selected area.
Screenshot a specific application
Command + Shift + 4 + Space Bar
A camera will appear, and you simply move it to the right window and click.
Empty the trash without confirming
Command + Option + Shift + Delete
Find typos in your text
Command + ;
Create a new folder
Command + Shift + N
Create a new email in Apple Mail
Command + Shift + I
Invert your desktop colors
Command + Control + Option + 8