From BlenderWiki

Extrude Dup

This tool repeatedly extrude selected elements a number of times, along the axis “normal” to the screen plane. There is no interactivity here, but you have three options to set/check before using this tool:

• The point of view you are using, as the extrusion will be aligned with it!
• The number of extruded duplications you want, controlled by the Steps numeric field (at the center of the group just above the Extrude Dup button, in the Mesh Tools panel).
• The distance along which you want your extruded copies to be spread, set in the Offset numeric field just to the right of the Extrude Dup button (defaults to 1.0 BU).

Once all this is set to your liking, just press Extrude Dup. Note that you will likely have to change your point of view to see anything new…