Command-Space: Show or hide the Spotlight search field.
Command-W: Closes the active window you are currently in.
Command-P: Print the current document.
Command-T: Open a new tab.
Command-S: Save the current document.
Command – Comma (,): You are working in an app, and you want to open the application's Preferences.
Command-G: Find the next occurrence of the item previously found.
Command-M: Minimize the front app window to Dock
Command-L: Center the cursor or selection in the visible area
Command-Option-D: Show or hide the Dock from within most apps.
Command-left/right arrows: Command and the left/right arrows to go back/forward on a page in the browser window 
Fn twice: Launch Dictation on your Mac, start speaking, and press fn once you've finished.
Command + Control + Q: shortcut to immediately lock your machine.
Command+D: Bookmark the current page.
Command+B: View bookmarks.
Command+J: View recently downloaded files.
Command+Delete: Send a selected file to the Trash.
Space: Preview the selected file.