The Null Cell is a part of the RedPower2 mod and allows two Redstone signals to cross a single block without interfering with each other. It is very handy when you are crossing two Redstone wires in a compact circuit space.
Recipe[]
The Null Cell in action.
Bugs[]
A Redstone signal through Redstone Dust will remain on after the power source has been removed (as of Tekkit 3.1.2, see image).
Powering the circuit from the Dust side enables a signal, as it should. Remove the signal source and the Dust remains powered.
A Redstone signal will carry through the device if one is using Redstone Dust on one side and Red Alloy Wire on the opposite side (as of Tekkit 3.1.2).