  We report measurements of $\Omega_M$, $\Omega_\Lambda$, and w from eleven
supernovae at z=0.36-0.86 with high-quality lightcurves measured using WFPC-2
on the HST. This is an independent set of high-redshift supernovae that
confirms previous supernova evidence for an accelerating Universe. Combined
with earlier Supernova Cosmology Project data, the new supernovae yield a
flat-universe measurement of the mass density $\Omega_M=0.25^{+0.07}_{-0.06}$
(statistical) $\pm0.04$ (identified systematics), or equivalently, a
cosmological constant of $\Omega_\Lambda=0.75^{+0.06}_{-0.07}$ (statistical)
$\pm0.04$ (identified systematics). When the supernova results are combined
with independent flat-universe measurements of $\Omega_M$ from CMB and galaxy
redshift distortion data, they provide a measurement of
$w=-1.05^{+0.15}_{-0.20}$ (statistical) $\pm0.09$ (identified systematic), if w
is assumed to be constant in time. The new data offer greatly improved color
measurements of the high-redshift supernovae, and hence improved host-galaxy
extinction estimates. These extinction measurements show no anomalous negative
E(B-V) at high redshift. The precision of the measurements is such that it is
possible to perform a host-galaxy extinction correction directly for individual
supernovae without any assumptions or priors on the parent E(B-V) distribution.
Our cosmological fits using full extinction corrections confirm that dark
energy is required with $P(\Omega_\Lambda>0)>0.99$, a result consistent with
previous and current supernova analyses which rely upon the identification of a
low-extinction subset or prior assumptions concerning the intrinsic extinction
distribution.
