  This paper reports the discovery and characterization of the transiting hot
giant exoplanet Kepler-17b. The planet has an orbital period of 1.486 days, and
radial velocity measurements from the Hobby-Eberly Telescope (HET) show a
Doppler signal of 420+/-15 m.s-1. From a transit-based estimate of the host
star's mean density, combined with an estimate of the stellar effective
temperature T_eff=5630+/-100 K from high-resolution spectra, we infer a stellar
host mass of 1.061+/-0.067 M_sun and a stellar radius of 1.019+/-0.033 R_jup.
We estimate the planet mass and radius to be Mp=2.450+/-0.114 M_jup and
Rp=1.312+/-0.018 R_jup and a planet density near 1.35 g.cm-3. The host star is
active, with dark spots that are frequently occulted by the planet. The
continuous monitoring of the star reveals a stellar rotation period of 11.89
days, 8 times the the planet's orbital period; this period ratio produces
stroboscopic effects on the occulted starspots. The temporal pattern of these
spot-crossing events shows that the planet's orbit is prograde and the star's
obliquity is smaller than 15 deg. We detected planetary occultations of
Kepler-17b with both the Kepler and Spitzer Space Telescopes. We use these
observations to constrain the eccentricity, e, and find that it is consistent
with a circular orbit (e<0.0011). The brightness temperatures of the planet the
infrared bandpasses are T_3.6um=1880+/-100 K and T4.5um=1770+/-150 K. We
measure the optical geometric albedo A_g in the Kepler bandpass and find A_g =
0.10+/-0.02. The observations are best described by atmospheric models for
which most of the incident energy is re-radiated away from the day side.
