Two kinds of cavity solitons, stable circular domain walls (droplets) and dark ring cavity solitons, are presented in models of vectorial Kerr resonators and degenerate optical parametric oscillators. These structures are universal in systems with two equivalent homogeneous states and are found for parameter values close to those of a modulational instability of a flat front. We show that stable droplets nucleate out of dark ring cavity solitons and that in some systems there are regimes in which they coexist.