When I'm working on a story I often make use of a number of reference sources. Some of these might be of interest to other writers (and non-writers). Some of these are well known, such as Wikipedia, but many are not as widely shared, so I thought I'd link to them here.
This is a great resource for calculating travel times and relativistic speeds for journeys to celestial bodies. I've used it a lot in working out timelines in the sequel to Mathematics Of Eternity. So much so that I've mirrored it on my own site (with Nathan's gracious permission).
The atomic rockets site is an amazing wealth of information on space travel, possible future technologies, and the science of space. It does a great job of dispelling many of the myths perpetuated by movies and TV. Essential reading for anyone wanting to inject reality into their work.
Very useful calculators for acceleration, artificial gravity in a rotating cylinder, time-dilation, and much more. There are also a number of useful and interesting articles on the site relevant to writing SF.
A fantastic planetary surface generator that also creates realistic physical data such as mass, gravity etc. There are many other generators on the homepage here that may be of interest.
A great youtube channel with videos discussing many aspects of science, space exploration and possible future technologies.