Free Parallize Walls Add-in for Revit (Updated)

From the CASE Apps website...

Parallize Walls will align a target wall to a reference object using the midpoint of the centerline of the target wall.

First Selection: Detail Line, Model Line or Wall element.

Second Selection: Target Wall element.

There's more information available on the CASE Apps website.

Update 5/8/2016: CASE and the CASE Apps website are no more but, courtesy of WeWork, the CASE Apps are now open source. They may be accessed on GitHub.