  We discuss dark energy models which might describe effectively the actual
acceleration of the universe. More precisely, for a 4-dimensional
Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) universe we consider two
situations: First of them, we model dark energy by phantom energy described by
a perfect fluid satisfying the equation of state $P=(\beta-1)\rho$ (with
$\beta<0$ and constant). In this case the universe reaches a ``Big Rip''
independently of the spatial geometry of the FLRW universe. In the second
situation, the dark energy is described by a phantom (generalized) Chaplygin
gas which violates the dominant energy condition. Contrary to the previous
case, for this material content a FLRW universe would never reach a ``big rip''
singularity (indeed, the geometry is asymptotically de Sitter). We also show
how this dark energy model can be described in terms of scalar fields,
corresponding to a minimally coupled scalar field, a Born-Infeld scalar field
and a generalized Born-Infeld scalar field. Finally, we introduce a
phenomenologically viable model where dark energy is described by a phantom
generalized Chaplygin gas.
