Microplasmas refer to electric discharges created in very small geometries able to operate in DC mode at high pressure without glow to arc transition. The recent and considerable interest in microplasmas is due to their unique properties in term of discharge stability and power loading. A microplasma configuration which has proven to be stable at atmospheric pressure and up to a power density of some 100 kW/cm3 is the Micro Hollow Cathode Discharge (MHCD) developed by Schoenbach and coworkers. MHCDs are created by applying a voltage between two closely spaced hollow electrodes separated by a dielectric layer. The thickness of the dielectric and the diameter of the hole are both on the order of some 100's microns. These MHCDs can be used as plasma cathodes for generating a diffuse discharge between the MHCD cathode and a third positively-biased electrode placed some distance away. This is the so-called Micro Cathode Sustained Discharge (MCSD) configuration, which can be operated as a non self-sustained discharge. In that mode, the MCSD appears as a unique tool for producing, at high pressure, large fluxes of O2(a1Δg) metastable states which cannot be efficiently produced in classical self-sustained discharges. Based on experimental works performed by our group and on modeling studies done by Pitchford and coworkers at Toulouse University, this paper summarizes the properties of the plasmas generated by the MHCD and the MCSD, with an emphasis on the new fascinating opportunities for the production of O2(a1Δg) metastable states by electrical discharges in high pressure rare gas-oxygen mixtures.