Water mist has been around for a long time. Its the same principle as a traditional sprinkler head, except instead of using water pressures in the 50 to 150 psi range to create water droplets to penetrate a fire plume, it uses a high pressure pump and a nozzle to create the mist with a relatively small volume of water (comparably). The application is therefore geared to enclosed spaces. It has its limitations when it comes to flammable liquids fires depending on which standard or research project you are looking at.
High end water mist systems for ships, yachts, and shore side machinery spaces like gas turbine enclosures are not cheap but they are using sophisticated componentry, detectors, etc.
The clean agents on the market (Kidde Fireboy for example) are still the best choice for us IMO. A relatively small volume, self contained, with a simple detection system (fusible link for example). Not very $, compact, and proven for our small engine rooms if you have enough gas and calculated the protected volume properly which is not always done.
There are many other clean agent gases that came on the market after Halon was classified as an OD years ago. These can be used with humans which will allow them to breath as they egress the space. Most of the big computer rooms use these along with a dry, pre-action sprinkler system as the back up in case the gas (which is often single shot) fails to control the fire.
C02 is generally not used in normally occupied spaces because it is can kill people if they get stuck in the space.
Shoreside C02 use in non occupied machinery spaces, and to protect flammable liquids are still used; however, the fire protection community has learned that C02 is not as effective as originally thought for FL fires...long story.