Docky is a full fledged dock application that makes opening common applications and managing windows easier and quicker.

Avant Window Navigator (AWN or Awn) is a dock-like bar for Linux, which sits on an edge of a user's screen and tracks open windows.

Plank is meant to be the simplest dock on the planet.

Cairo-Dock is a desktop interface that takes the shape of docks, desklets, panel, etc.

tint2 is a system panel/taskbar for Linux. It is described by its developers as "simple, unobtrusive and light".