SponsorBlock

SponsorBlock is a free and open-source browser extension that skips segments of YouTube videos. Users make them, which other users can vote on. It is recommended on Firefox's add-on store. , it has over 175,000 downloads on Firefox and over 600,000 downloads on Google Chrome.
History.
Only sponsorships could be skipped until more categories (such as self-promotion or intermissions) were added in June 2020. An update in January 2022 added support for marking whole videos as sponsorships.
Usage.
Users submit segments of videos and choose a category for them. Those segments are automatically skipped. When a segment is skipped, a small pop-up appears for a few seconds to allow the user to vote on it or "unskip" it. The number of segments each user has submitted and skipped is tracked.
Reception.
The extension was on Mozilla's Extension Spotlight on July 21, 2020, and is the highest rated recommended Firefox extension.