Attractive interactions between like-charged aggregates (macromolecules, colloidal particles, or micelles) in solution due to electrostatic correlation effects are revisited. The associated phenomenon of phase separation in a colloidal solution of highly charged particles is directly observed in Monte Carlo simulations. We start with a simple, yet instructive, description of polarization effects in a "cloud" of counterions around a single charged aggregate and show how the ion-ion correlations can be mapped onto a classical analogue of the quantum-mechanical dispersion force. We then extend our treatment to the effective pair interaction between two such aggregates and provide an analysis of different interaction regimes, based on a simple coupling parameter. By computing the potential of mean force, we illustrate the physics behind the crossover between the regimes of pure repulsion and attraction with increasing counterion valency. Finally, we turn to semi grand NpT simulations of the corresponding bulk systems where mono- and multivalent ions can exchange with an external reservoir. Thus, the coagulation and phase separation phenomena, widely observed and used in real-life applications, are directly studied in these computer simulations.