Here’s Cory’s impressively crowded Dock before he added spacers...
...and afterward, those lovely little gaps create logical groupings of kinds of apps.
The Dock is a great place for your most-used applications, but it can get a little crowded. With a Terminal command, you can add spacers to your Dock to organize things a little better. Open Terminal (Applications > Utilities), then type the following command:
defaults write com.apple.dock persistent-apps -array-add '{"tile-type"="spacer-tile";}'
Type the command once for each space you’d like to add, then type the following to restart the Dock:
killall Dock
In the new Dock, you’ll see the spacers to the right of your applications. Drag spacers around to create “sections” of applications in your Dock. To remove a spacer, simply drag it off the Dock.
Source: http://www.maclife.com/article/howtos/how_put_spacers_your_dock_organize_apps
More:
Six Ways to Download YouTube Video on Mac