And to be almost complete, once you entered X or Y or Z using keyboard to specify along which direction the selection should be extruded, you can enter the length of the extrusion with the numpad. E.g. : type "3", ".", "0" and "4" to extrude by 3.04 Blender units.
Pressing once X (or Y or Z) will extrude along global X axis. Pressing twice will extrude along local axis. Pressing a third time will cancel axis selection.
I hope we were clear
