There are a number of high pressure water mist fire suppression systems. The underlying principle is they use water broken into tiny droplets by forcing the water through special nozzles. This greatly increases the total surface area of the water which has the effect of greatly increasing the water cooling capacity. These systems use 10% of the water that a normal fire sprinkler system would use to achieve the same effect.
The technique was developed in the shipping industry where it is widely used for engine room fire suppression systems. It is not a new technology, so most of the kinks have been ironed out. There are a range of systems including pumped and stored gas (e.g. Nitrogen) used to deliver the water. Some are room based there even ones which operate in equipment cabinets.
The mist is non-toxic. It is after all just distilled water. So it is less hazardous than fire suppressant gases to personnel in the area of operation. The non-ionic water and tiny particles are not good electricity conductors so it can be used in the proximity of live equipment. The mist also suppresses smoke and reduces soot damage from the fire.
One example of this is the Hi-Fog system marketed by Marioff; there are other systems which can be easily discovered through Google.