Basically an old 1.x version of Jasmine, bolted to a test runner, a JSDom environment, and a ton of mostly working automocking magic. It's glacially slow and kind of buggy, and in my experience most projects using React use Mocha, not Jest. And I haven't run across any using actual Jasmine (Jasmine and Jest are neither compatible nor interchangable).
As a general rule, I'd say node-based projects are heavily mocha based, whereas browser-based projects are much more mixed. CoffeeScript based projects seem especially likely to use Mocha over Jasmine; Angular (as you noted) has a strong preference for Jasmine due to their Protractor tool, etc. QUnit seems to be used by a lot of libraries and frameworks, but to have almost no usage among projects; I've never seen QUnit tests in the wild except in framework/library repos.
And as a final data point, note that on NPM, the main mocha package has almost 5 times the weekly downloads of the main jasmine package.
It was extremely straightforward to integrate with Mocha, though. We just needed to stub out some of the node.js-specific code and provide a very small wrapper to provide our own module loader and reporting plugins. There is an active effort to split up Mocha into multiple modules in order to make it more portable, which would make the stubbing unnecessary, too.
Haven't found an instance of Mocha. Most job postings I've seen indicate that Jasmine is the dominant test framework.