Any reversible physical process is an energy storage technology.
An interesting take from Technology Connections.
In this one, he reminds us that simple management of heat in our homes can amount to a simple and immediately available method to distribute energy use from the hours of peak demand.
Crooked Timber says: "energy storage is a solvable problem [...] concerns about the variability of wind and solar power will come to nothing in the end".
Moving rocks up and down former mine shafts could be a workable energy storage system.