Make an Impact!
Tool for simulating the impact of an asteroid or comet with planets and moons in the Solar System.
http://janus.astro.umd.edu/astro/impact/ - Details

Rio Cuarto Impacts

http://www.crateres.com/index.htm - Details

Terrestrial Impact Sites

http://www.uwgb.edu/dutchs/planets/impact1.htm - Details

SL-9 The Impact - NASA

http://nssdc.gsfc.nasa.gov/planetary/impact.html - Details

Spaceguard Croatia

http://www.astro.hr/spaceguard/ - Details

The Increasing Rate Of Discovery In Asteroids Armagh Observatory

http://www.arm.ac.uk/neos/ - Details

Simulation Of Comet Impact With Earth
Real (not reel) deep impacts: Sandia scientists predict what an asteroid strike would look like, really.
http://www.sandia.gov/media/comethit.htm - Details

Shoemaker - Levy 9 Jupiter Impact
Fragments of comet P/Shoemaker-Levy 9 collided with Jupiter on July 16-22, 1994. The results were spectacular.
http://www.solarviews.com/eng/impact.htm - Details

NEO Deflection Technology (Fact & Fantasy)

http://star.arm.ac.uk/slides/nh2/NEO-Deflection-Technology.html - Details

list of identified impact craters on Earth (plain text)

http://www.xs4all.nl/%7Ecarlkop/kraters.html - Details

Current Impact Risks from NASA

http://neo.jpl.nasa.gov/risk/ - Details

Armagh Observatory's NEO Section

http://star.arm.ac.uk/impact-hazard/ - Details

Small Asteroids Encounters List
SAEL - Small Asteroids Encounters List ( Asteroids with H > 22.0 )
http://www.brera.mi.astro.it/sormano/sael.html - Details

The Planetary Society NEO Pages

http://www.planetary.org/explore/topics/near_earth_objects/ - Details

NEOs - Origin, Collisional Rate & Actuarial Risk - Mark Bailey

http://www.arm.ac.uk/paseg/ - Details

Shoemaker - Levy 9 Post Impact Q&A

http://www.isc.tamu.edu/~astro/sl9/cometfaq2.html - Details

Earth Impact Effects Program

http://www.lpl.arizona.edu/impacteffects/ - Details

The Australian Spaceguard Survey
asteroids, comet, mass extinction, astrobiology
http://users.tpg.com.au/users/tps-seti/spacegd.html - Details

AIAA Position Paper (1995) - David Morrison

http://impact.arc.nasa.gov/gov_aiaa95.cfm - Details

Future Asteroid Interception Research
Assist in the search for asteroids, looking for threatening space rocks is a seriously underfunded science, this website helps with the hunt.
http://www.fair-society.org/index.htm - Details