  We present new constraints on the evolution of dark energy from an analysis
of Cosmic Microwave Background, supernova and X-ray galaxy cluster data. Our
analysis employs a minimum of priors and exploits the complementary nature of
these data sets. We examine a series of dark energy models with up to three
free parameters: the current dark energy equation of state w_0, the early time
equation of state w_et and the scale factor at transition, a_t. From a combined
analysis of all three data sets, assuming a constant equation of state and that
the Universe is flat, we measure w_0=-1.05+0.10-0.12. Including w_et as a free
parameter and allowing a_t to vary over the range 0.5<a_t<0.95 where the data
sets have discriminating power, we measure w_0=-1.27+0.33-0.39 and
w_et=-0.66+0.44-0.62. We find no significant evidence for evolution in the dark
energy equation of state parameter with redshift. Marginal hints of evolution
in the supernovae data become less significant when the cluster constraints are
also included in the analysis. The complementary nature of the data sets leads
to a tight constraint on the mean matter density, Omega_m and alleviates a
number of other parameter degeneracies, including that between the scalar
spectral index n_s, the physical baryon density Omega_bh^2 and the optical
depth tau. This complementary nature also allows us to examine models in which
we drop the prior on the curvature. For non-flat models with a constant
equation of state, we measure w_0=-1.09+0.12-0.15 and Omega_de=0.70+-0.03. Our
analysis includes spatial perturbations in the dark energy fluid, assuming a
sound speed c_s^2 =1. For our most general dark energy model, not including
such perturbations would lead to spurious constraints on w_et which would be
tighter by approximately a factor two with the current data. (abridged)
