There are a lot of calculators, simulators, plotters, visualizers, and libraries of bits of code and models and such online. Many of us are aware of a bunch of them, and they are great learning tools and aids for projects of all kinds. So, in the spirit of other very popular posts here, I'd like to propose we collect a list of them and update it as new things are found.
In particular, like these questions:
Resources and references on the topic of space exploration
Collection of space exploration related infographics
The Resources and references list has a set of answers that split the material into sections, each of which is a community wiki (people collaborate to add material to the answer and the usual point system doesn't apply). That seems like it would be effective here too.
I started two answers, one for tools that can be used in a browser, and one for things that are downloaded for use. Possibly it would be useful to distinguish between free and for sale material too.
