Planet
The smallest known planet is PSR B1257+12A, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar. Its mass is roughly half that of the planet Mercury. The smallest known planet orbiting a main-sequence star other than the Sun is Kepler-37b, with a mass (and radius) slightly higher than that of the Moon.
HD 49674
At the time of discovery of the planet HD 49674 b in 2002, it was the least massive planet known, very close to the boundary between sub-Jupiter mass and Neptune-mass at 0.1 M. This planet orbits very close to the star.