Very simple two, three, and four way gang valves. Plug a standard airline into one end and these split the air between the desired amount of airlines. This has a clip on it as well so you can attach it to the back of your tank.
Rivers to Reef says: Gang valves will help split the air flow from a pump among multiple lines. Whether or not a pump is going to have the power to run multiple air lines is a complicated issue. (You need to take into account water depth and the resistance of what you are running on the end of the air line). If you are in doubt, buy an air pump that is one size bigger than is recommended.