Curve Offset In 3D Space is an add-on from Luxuy Store for Blender 2.7x that offsets a selected bezier spline in 3D space. It works in curve edit mode and currently supports the bezier spline type only.
You can offset a curve toward the cursor location, either passing through the cursor point or by setting a specific offset distance. A steps option lets you apply multiple offsets in one operation, and the tool can offset through a 3D point precisely, currently for straight segments only.
Installation places the Python file in the scripts/addons/ directory, after which pressing F8 refreshes Blender and the add-on can be enabled from the add curve category in the user preferences. In curve edit mode, select a spline and use the W-key menu or the Ctrl+R hotkey, then set the distance and steps. It is released under the GPL license and its developer is a Blender Development Fund contributor.