  The generalized Chaplygin gas is characterized by the equation of state p = -
A/rho^alpha, with alpha > -1 and w > -1. We generalize this model to allow for
the cases where alpha < -1 or w < -1. This generalization leads to three new
versions of the generalized Chaplygin gas: an early phantom model in which w <<
-1 at early times and asymptotically approaches w = -1 at late times, a late
phantom model with w \approx -1 at early times and w -> - \infty at late times,
and a transient model with w \approx -1 at early times and w -> 0 at late
times. We consider these three cases as models for dark energy alone and
examine constraints from type Ia supernovae and from the subhorizon growth of
density perturbations. The transient Chaplygin gas model provides a possible
mechanism to allow for a currently accelerating universe without a future
horizon, while some of the early phantom models produce w < -1 without either
past or future singularities.
