There are a bunch of external resources that are either used very often by this community or are directly related to it. This post contains the most relevant ones to make them more discoverable for new users.
Here is how this will work: put each resource in a separate answer, linking to it at the top. You can use the answer body to explain what it is and how it's useful. Also edit a link into the list below for quick access. The idea is that the votes will let the most useful resources bubble up to the top. If an answer gets too many downvotes, it will probably be removed to keep the list useful and the noise low.
Please also make sure that the type of resource isn't covered by the related meta posts at the bottom (for example links to online testing environments).
Quick access links
Some other meta posts that list external resources of more specific purpose: