This paper presents a working silicon-based MEMS electrostatic transducer for harvesting and converting the energy of vibrations into electrical energy. The transducer is fabricated in a batch silicon-on-glass process. Two designs, for 1D and 2D vibrations, are addressed. The 1D harvester demonstrates between 60 and 100 nW of mechanical-to-electrical power conversion at 250 Hz when implemented in a classical DC/DC converter circuit. We also propose a new simple approach for the calculation of the maximum power that can be generated from a spring-mass system excited with a sinusoidal force, based on an electrical impedance network analogy. This paper ends with a comparison between several working electrostatic harvesters based on a novel figure of merit.