Especially if you are a sci-fi writer:
List of Known Trans-Neptunian Objects
(and other outer solar system objects)
A list of asteroids, planetoids, Kuiper Belt Objects and such floating around past Neptune. Includes basic orbital elements, brightness, diameter and any known moons that they may have.
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Similar to previous.
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Current Impact Risks
Keep tabs on the rocks out there that will KILL US ALL.
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JPL Orbit Diagrams
Related to the Impact Risks page, this provides Java-based 3-D orbit diagrams of asteroids and planets. Zoom in and out, rotate around, animate them forward and backwards in time, watch how DOOM WILL COME AND SMACK EARTH UPSIDE THE HEAD.
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Atomic Rockets
If you want to write science fiction involving spacecraft, spaceflight or space battles, and you want it to be scientifically plausible, and you don’t have a relevant college degree… you’d be an unforgivable chump if you don’t pay a whole lot of attention to this website.
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Extra-Solar Planets Catalog
There are more of them than you might think… 708 as I write this. Cataloglists planets based on discovery means… direct imaging, microlensing, astrometry, etc.
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Earth Impact Effects Program
Enter the basic data of the DOOM-BRINGING rock or comet, and find out just how big of a hole it’s going to make in the Earth, and just how big that blast is going to be.
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Did I miss any? Yes. yes, I did. Feel free to add relevant links to the comments. I will add more to this listing as they occur to me (or commented links that I think are especially good).