Post your links that might be helpful for the development of games based in space - whether it is for art, procedural generation or just informations about our universe. I'll keep the original post updated as more people post their links, and try to categorize them. I'll start with mines.
Resources: textures/models/concept art or useful photos
Concept art on Star Trek's spaceships Astronomical images: lots of photos.. you have to dig.
NASA Breakthrough Propulsion Physics Project Art: concepts about distortions due to propulsion
Global Topographic Data: to get very detailed heightmaps of a region of Earth.
Planetary maps: textures to map on planets. It has a few moons textures (Io, Europa, Ganymede, Callisto).. which are hard to find.
Space and technology Generators: to generate descriptions randomly. Funny for 5 mins.
Being an Informal Examination of Offensive Systems for Use in Deep-space Combat: a review of popular space weapons and how realistic they are.
Creating 3D planets in photoshop: a short tutorial to create a planet background.
Making a realistic starfield: the best photoshop tutorial to make a good space background, ever.
Texture maps of Earth and Planets: 2500x1250 textures of some planets
Blue Marble (NASA Earth dataset): 8192x4096 textures of Earth
Celestia Earth textures: 43200x21600 textures of Earth (HUGE downloads!)
Astronomy picture of the day: every day an image/photo
Science and nature: space (BBC site): a lot of interesting/comprehensible infos for the amateur
Planetary image maps: generally 1024x512 textures, bumps, specular and cloud maps of most planets. NEW
sci.astro.research: a newsgroup for astrophysics research NEW
A collection of space links: a lot of useful links but you have to dig NEW
Space Art: a good inspiration NEW
Solar Voyager Tutorials: very useful from the artist's point of view. Be sure to check out the How to make a planet tutorial. NEW
The NASA's Bluemarble ftp site (direct link): This is the ULTIMATE link if you're looking for high-resolution Earth datasets. The ftp site is slow to connect but works. You can get heightmaps, texture maps, cloud maps, normal maps and light maps of the full Earth at a resolution of 1 km (43200x21600 texels).
NEW About books, movies or games
Logos for spaceships: related to the Empire in Foundation by I. Asimov
Known Spacer worlds: the worlds found in Foundation by I. Asimov
The Grand List of Overused Science Fiction Clichés: worth a read (and a laugh) if you design a space-based game
Sirroco Station: fan site for the game serie "Elite" of D. Braben
A list of space-based games.. commercial or not: one of the biggest list i know speaking of the "competition".
Watch the Elegant universe: a serie of videos explaining space and astrophysics NEW
Astrophysics
Is the speed of light constant? NEW
Frequently Asked Questions in Cosmology NEW
Astronomical Constants and Tables: a table with the value of important constants, as well as planetary orbital elements, all summed up in a single page NEW
Galaxies/Universe
A list of nearby galaxy groups (within 100 millions LY)
Atlas of the universe: infos about our universe and its structure
Nebulas
Volume visualization of the Orion Nebula: if you wondered how a nebula looks in 3D, this is an absolute "must see".
Making a nebula texture in photoshop: extremely interesting if you plan to generate nebulas procedurally.
The Orion Nebula (photoshop tutorial): another tutorial to make a very realistic nebula texture in photoshop
Stars/Stars systems
A scale model of the solar system: with a nice applet showing orbits.
Wikipedia: what's a binary star How to compute planetary positions:
The ultimate link for the programmer: how to generate the positions of the planets in our solar system given a specific date
Globular Clusters: a list of known stars clusters.
The Magnitude System: nagivate the other pages on the same site, you'll find tons of infos, often understandable for the amateur, about stars properties (not only magnitude)
JPL Solar System Simulator: or the NASA's own simulator. Can render our solar system from any viewpoint. Uses a web interface.
ARVAL Catalog of Guide Stars: a list of "common" names for known stars.
The 100 nearest star systems: data for the.. the 100 nearest star systems (what did you expect?).
Data for important stars: 333 stars, and one page of description with data for each.
How Stars are Named: stellar nomenclature.
A solar system generator with RPG rules: extremely useful but uncomplete
Notable Nearby Stars: a mine of informations about nearby systems NEW
Computing planetary positions, a tutorial with worked examples NEW
Planets
Keplerian/Orbital elements and orbits: interesting.. for its diagrams.. or for the expert.
The living Earth: a short description of how colors are affected by the atmosphere for aerial views.
Fractal Models of Natural Phenomena: some infos on how to use fractals/procedural noise to generate planet's heightmaps
Earth rendering in Maya: has very good infos on how to apply different layers of textures and to blend them to make a realistic Earth.
Guide to extrasolar planets: infos (and VRML presentations) of extrasolar planets/systems. NEW
Computing planet positions using mean orbital elements: equations and code to compute planetary positions in 3D NEW
Use defined astronomical functions in Excel: VBA code for computing planetary positions too NEW
An applet to visualize orbits: with more on the same site NEW
The Nine Planets: important data and infos about planets, and harder to find, their satellites! NEW
3D Engines or software
Magrathea Simulation Engine Geofusion: looking very good but hem.. you need the satellite photos that go with it 
Celestia: an astronomy software that contains a large set of accurate space objects.
Hallo Northern Sky software page: a free planetarium for Windows NEW
Stellarium: a competitor of Celestia NEW
Y.
PS.: i still have more at work but i'm currently in vacation.. you'll have to wait 2 weeks before i can update this list with my other links.
[edit by dwarfsoft] fixed the first link in the "Planets" section ... one too many <a href=" 's ;) [/edit]
