Does the Y-Δ transform suffice for every resistor network?
Given a network of resistors placed on the edges of a cube and connected at the corners, what is the resistance of this network across the body diagonal of the cube?

Take a look at this image


(source: lecturedemonstrations at web.physics.ucsb.edu)  

Try turning Y into Δ and Δ into Y a couple of times and see if that helps you solve the problem.

The image is from here where you can also find the solution.