This script uses the concept of support edges, or the inserting of new edges
parallel to the selected edges.
First go to User Preferences->Addons and enable the ParEdge addon in the
Go to EditMode, select a closed edge path without corners (an edgeloop or
part of an edgeloop)and invoke
the addon (button in the Mesh Tool panel). Enter a distance (positive
or negative) with the slider in
the Mesh Tools panel or leftclick-drag from left to right to
interactively choose the parallel distance. Select "Both Sides" on the
panel to insert edges on both sides.
Select "Endpoint quads" if you want to have edgepath endpoints "capped".
Press the right mouse button to cancel operation or ENTER to accept
The tool will remember the last set distance and the "Both Sides" setting
for the next ParEdge operation.
If you wish to hotkey ParEdge:
In the Input section of User Preferences at the bottom of the 3D View >
Mesh section click 'Add New' button.
In the Operator Identifier box put 'mesh.paredge'.
Assign a hotkey (I used Alt+T).
Save as Default (Optional).