Solar System

IAU: Minor Planet Center - The official body that deals with astrometric observations, and orbits of minor planets, asteroids, and comets.
www.cfa.harvard.edu/iau/mpc.html

National Space Science Data Center - Provides access to a wide variety of astrophysics, space physics, solar physics, lunar, and planetary data from NASA space flight missions, in addition to selected other data, models, and software.
nssdc.gsfc.nasa.gov/

PDS - Planetary Data System - Solar system exploration science data archive, maintained at JPL.
pds.jpl.nasa.gov/