Due to their particular properties (low emittance, short duration, and large number density), the beams of multi-MeV protons generated during the interaction of ultraintense (I > 1019 W/cm2) short pulses with thin solid targets are suited for use as a particle probe in laser-plasma experiments. When traversing a sample, the proton density distribution is, in general, affected by collisional stopping, scattering and deflections via electromagnetic fields, and each of these effects can be used for diagnostic purposes. In particular, in the limit of very thin targets, the proton beams represent a valuable diagnostic tool for the detection of quasi-static electromagnetic fields. The proton imaging and deflectometry techniques employ these beams, in a point-projection imaging scheme, as an easily synchronizable diagnostic tool in laser- plasma interactions, with high temporal and spatial resolution. By providing diagnostic access to electro-magnetic field distributions in dense plasmas, this novel diagnostics opens up to investigation a whole new range of unexplored phenomena. Several transient processes were investigated employing this technique, via the detection of the associated electric fields. Examples provided in this paper include the detection of pressure-gradient electric field in extended plasmas, and the study of the electrostatic fields associated to the emission of MeV proton beams in high-intensity laser-foil interactions.