Formerly a frontend for GNOME Do (Do), and now its own application. Docky is a dock intended for use in the GNOME desktop environment.
Docky started as a frontend for GNOME Do; it retained GNOME Do's functionality of allowing the user to search for items and perform actions on them, but also allowed apps to be pinned to the dock.
The modern version of Docky is no longer based on GNOME Do, but is a full-fledged dock application in its own right for the GNOME desktop. It features application shortcuts with quicklist support, as well as a variety of widgets and themes.
(Screenshot from wiki.go-docky.com)
Docky is also the precursor to the Plank, which was rewritten in Vala instead of Mono and is used as the dock for elementary OS's Pantheon desktop environment.