IAU: Minor Planet Center
We are the official body that deals with astrometric observations and orbits of minor planets (asteroids) and comets.
http://cfa-www.harvard.edu/iau/mpc.html

The Asteroid Orbital Elements Database
astorb.dat is an ASCII file of high-precision osculating orbital elements, ephemeris uncertainties, and some additional data for all the numbered asteroids and the vast majority of unnumbered asteroids (multi-apparition and single-apparition) for which it is possible to make reasonably determinate computations.
ftp://ftp.lowell.edu/pub/elgb/astorb.html

Small-Body Orbital Elements
from JPL/NASA. Provides access to orbital elements for the numbered asteroids, unnumbered asteroids, and comets contained in JPL's DASTCOM database.
http://ssd.jpl.nasa.gov/sb_elem.html

Minor Planet Mailing List Random Archive and FAQ
These pages contain information about the Minor Planet Mailing list as well as a number of interesting and important ideas for the minor planet observer.
http://www.bitnik.com/mp/

Minor Planet Center Orbit Database
Ftp access to the complete set of minor planet (asteroid) orbits generated and compiled by the International Astronomical Union's Minor Planet Center. MPCORB contains orbits for every object for which it is possible to make a reasonable prediction of where the object is. MPCORB currently contains over 30000 orbits.
ftp://cfa-ftp.harvard.edu/pub/MPCORB

Sky and Telescope's Asteroids Page
Ftp access to the complete set of minor planet (asteroid) orbits generated and compiled by the International Astronomical Union's Minor Planet Center. MPCORB contains orbits for every object for which it is possible to make a reasonable prediction of where the object is. MPCORB currently contains over 30000 orbits.
http://SkyandTelescope.com/observing/objects/asteroids/

Minor Planet
Dediced to minor planets and astronomical imageing. Contains information on minor planets, building observatories and robotic telescopes and remote systems.
http://www.minorplanet.org/