GitHub
The website where most of the world's software is stored, built, and shared. Much of the activity on it is public and dated.
What it tells you about a developer
A developer with a rich GitHub presence gives you verifiable evidence of real work: what they built, when, and with whom. You can check that evidence yourself, which is more than a resume line offers.
GitHub hosts code projects and records every change to them, with an author and a timestamp attached. Over 100 million developer accounts use it, and nearly all major open source software lives there. For hiring, it is the largest public record of what developers have actually built.