  We report the discovery of PSR J1906+0746, a young 144-ms pulsar in a highly
relativistic 3.98-hr orbit with an eccentricity of 0.085 and expected
gravitational wave coalescence time of 300 Myr. The new pulsar was found during
precursor survey observations with the Arecibo 1.4-GHz feed array system and
retrospectively detected in the Parkes Multibeam plane pulsar survey data. From
radio follow-up observations with Arecibo, Jodrell Bank, Green Bank, and
Parkes, we have measured the spin-down and binary parameters of the pulsar and
its basic spectral and polarization properties. We also present evidence for
pulse profile evolution, which is likely due to geodetic precession, a
relativistic effect caused by the misalignment of the pulsar spin and total
angular momentum vectors. Our measurements show that PSR J1906+0746 is a young
object with a characteristic age of 112 kyr. From the measured rate of orbital
periastron advance 7.57+/-0.03 deg/yr, we infer a total system mass of
2.61+/-0.02 Msun. While these parameters suggest that the PSR J1906+0746 binary
system might be a younger version of the double pulsar system, intensive
searches for radio pulses from the companion have so far been unsuccessful. It
is therefore not known whether the companion is another neutron star or a
massive white dwarf. Regardless of the nature of the companion, a simple
calculation suggests that the Galactic birth rate of binaries similar to PSR
J1906+0746 is 60/Myr. This implies that PSR J1906+0746 will make a significant
contribution to the computed cosmic inspiral rate of compact binary systems.
