  We investigate the possibility of constraining dark energy with the
Integrated Sachs Wolfe effect recently detected by cross-correlating the WMAP
maps with several Large Scale Structure surveys. In agreement with previous
works, we found that, under the assumption of a flat universe, the ISW signal
is a promising tool for constraining dark energy. Current available data put
weak limits on a constant dark energy equation of state w. We also find no
constraints on the dark energy sound speed c_e^2. For quintessence-like dark
energy (c_e^2=1) we find w<-0.53, while tighter bounds are possible only if the
dark energy is ``clustered'' (c_e^2=0), in such a case -1.94<w<-0.63 at
2-sigma. Better measurements of the CMB-LSS correlation will be possible with
the next generation of deep redshift surveys. This will provide independent
constraints on the dark energy which are alternative to those usually inferred
from CMB and SN-Ia data.
