Vulnerability Lab

Vulnerability lab is a submission-and-disclosure platform that uses a team of in-house experts to vet high-profile vulnerabilities, but also accepts submissions on less critical/lower-profile bugs. One of their site's features actually involves receiving reports for critical vulnerabilities that a researcher might not want to submit directly and acting as a point of contact and third-party broker for the researcher with the affected company.

Like HackerOne, it publicly discloses bug reports after a window of time has elapsed, and is a useful reference for beginners looking to better understand the form of bug reports, and methods for discovering and reporting common vulnerabilities. Their public index of vulnerabilities is also tagged with the type of system each bug was found on, making it a nice resource when you're trying to get a sense of application-specific problems.