Every operating system has its own hidden quirks that aren't obvious at first, only way to usually find them is to read the manual but really no one ever reads the manual thoroughly. When you buy a Mac, you aren't even given a manual in the box, you have to go find it on Apple's support site.
The Mac has always had some awesome tricks that not everyone knows. Here are some not so obvious things you can do with your Mac.
1. Ctrl + Cmd + Spacebar brings up the full emoji keyboard.
2. Cmd + ~ allows you to quickly switch through windows on the current application you're on.
3. Shift + Option will give you the ability to more finely adjust the volume.
4. Holding down Shift when changing the volume will mute the volume clicker.
5. Cmd + L will highlight the URL bar in your browser.
6. Cmd + Shift + 3 will take a screenshot.
7. Cmd + Shift + 4 will let you take a cropped screenshot.
8. Cmd + Shift + 4 + Spacebar will let you take a screenshot of a specific window.
9. Using 'Preview' you can digitally sign documents. Here's a tutorial.
10. Cmd + Option + Shift + V will give you the ability to paste text with formatting.
11. Option + Right-Click on some things will bring up an extra contextual menu.
12. Option + hitting on any of the keys above the numbers such as brightness or volume will bring up settings for that function.
13. Cmd + Spacebar will give you quick access to spotlight.
14. OS X has a built in dictionary. Simply highlight a word, right click then hit 'Look Up'. Trackpad users can do it by tapping on a word with three fingers.
15. Highlight multiple items then hit the Spacebar and it'll show them in Quick Look.
16. Cmd + H will quickly hide the windows for the current app you're on.
17. Alt + backspace deletes whole words instead of deleting single letters.
18. Holding down Shift while minimizing a window will minimize it in slow motion.
If you have any other awesome tricks to share, make sure to leave them in the comments below!