NOTE: This tip is taken straight from my original post on AlternativeTo.
To extract an icon from a Mac app, first open your Applications folder. Right-click the app with the icon that you want, and click "Get Info".
A window will pop up. Click on the little icon on the top-left of the "Get Info" window. Copy it by using the keyboard shortcut Option+C (or Ctrl+C if you're using a Windows keyboard).
Open Preview, the image viewer built into Mac OS X. From the menu bar on the top of your screen, go to File->"New from Clipboard". Preview will instantly recognize the icon that you copied and create a new image out of it. You won't even have to paste the icon.
From the menu bar, go to File->Export. Save the file as a .PNG file, which is the most common format for transparent images (like app icons).
Congrats. You now have a transparent, 512x512-sized app icon, saved in the PNG format. Of course, you probably could have found the same icon in Google Image Search, but where's the fun in that?
SEE ALSO: Location of Mac OS X system icons