When you start to write a science fiction story, assuming that the science is important to you, it's often good to have some handy online references for your research.
For example, if your story involves a star in the sky blowing up and a mad scramble for everyone to get to cover -- wouldn't it help to do some research and discover that the nearest star to the Earth aside from the sun is Proxima Centauri? And wouldn't it be good to know the distance (approximately 4.2 light years)? And that if you're seeing it in the sky, the event happened a little over 4 years ago? And that you shouldn't be describing the sound of the star's explosion because, not only does sound travel slower than light, it doesn't carry at all in space.
Even if you're nodding because you knew all this, you brilliant reader you, it doesn't help to double check right? Here then is this week's list of useful science fiction links.
Wikipedia - No brainer, right? And yes, I know that any one can edit this "free encyclopedia", so yes, one should take some of the data on this with a grain of salt. However, pertinent articles usually have good links to other reference sites, making wikipedia visits useful enough to mention.
Grading SF for Realism - Not a science resource per se, but a useful page that helps explain what hard science fiction is, and helps define the degree of science "hardness". It helps authors grappling with the problem of plausibility by coming to terms with the level of scientific accuracy they'd like to achieve.
Project Rho's Atomic Rockets - A must read for anyone interested in writing about space ships and space combat in the science fiction genre (and perhaps even in the science fantasy genre). Aspects of rocketship science, design, and problems are explained clearly, with some occasional dips into heavy science-speak and mathematics.