Command (⌘)-Spacebar: Opens Spotlight for searching for files
Command (⌘)-Tab: Shows open programs. If you hold down the Command and repeatedly press Tab, you’ll switch between all open programs. You can also quit applications by hitting the Q while the Command is still held down.
Command (⌘)-Q: Quit programs
Command (⌘)-Shift-3: Takes a screenshot of the desktop. More screenshot options here.
For even more shortcuts, check out this complete list from Apple
Tip! If you’re coming to the Mac from Windows, know that almost any shortcut you learned in the Windows environment will work on your Mac if you use the Command(⌘) key instead of the Control key.