Using two complementary X-ray galaxy cluster studies we present new
cosmological constraints on dark energy. Using Chandra measurements
of the X-ray gas mass fraction, fgas, we obtain a detection
of the effects of dark energy comparable in significance to recent
type Ia supernovae (SNIa) studies. Using X-ray luminosity function
(XLF) data from galaxy cluster surveys, we obtain the first
interesting determination of the dark energy equation of state from
measurements of the growth of cosmic structure in clusters. Both of
our experiments provide strong, independent support to the
ΛCDM paradigm.