Flywheels have gone high-tech and are used to store electricity as kinetic energy which is instantly available when needed. One major use is for power backups.
Beacon Power is a big player here, and their flywheels recently got certified by the California Independent System Operator (CAL-ISO) to be used for frequency regulation in the state grid. This is a seriously big deal.
Flywheels have several advantages over lead-acid batteries in that they have no hazardous materials in them and operate for years with little maintenance. Thus, long-term, they are cheaper than batteries, as well as being much greener.
You can create electricity with renewable sources like solar and wind, store it using flywheels, and thus be completely off the grid.