I was working on a sketch and wanted a construction line midway between two model edges. In the past I'd have placed one construction line spanning between the edges, then made the other construction line coincident with its midpoint and parallel to one of the edges, but today it occurred to me to Ctrl+select the construction line and the two model edges. Guess what? The "Symmetric" relation showed up as an option.
I've often used the "Symmetric" relation to use a fixed construction line to drive other sketch elements, but it never occurred to me that I could have the relation drive the construction line. Am I the only one that didn't know about that?