One of the most under-the-radar new features to hit OS X Yosemite was the addition of extensions. Action extensions are particularly nice, because they allow apps to interface with other apps directly.
Some app developers, such as Pixelmator ($29.99 on the Mac App Store), have brought third-party extensions to the Mac, which demonstrates how beneficial extensions can be. Watch our video that showcases Pixelmator's Repair Tool extension to see what I mean.