  We consider Friedmann cosmologies with minimally coupled scalar field. Exact
solutions are found, many of them elementary, for which the scalar field energy
density, rho_f, and pressure, p_f, obey the equation of state (EOS)
p_f=w_f\rho_f. For any constant |w_f|<1 there exists a two-parameter family of
potentials allowing for such solutions; the range includes, in particular, the
quintessence (-1<w_f<0) and `dust' (w_f=0). The potentials are monotonic and
behave either as a power or as an exponent for large values of the field. For a
class of potentials satisfying certain inequalities involving their first and
second logarithmic derivatives, the EOS holds in which w_f=w_f(\f) varies with
the field slowly, as compared to the potential.
