Relatedly, if you want to skim videos to find relevant/interesting parts, this is incredibly useful: https://github.com/igrigorik/videospeed
That being said, Youtube itself is pretty good at this, with the shift+> hotkey, and the recent addition of 1.75 which is the sweet spot for me.
EDIT: What I fantastic extension, has all the options I could want (exclusion list, custom hotkeys, hide by default, etc).
In fact, writing this comment inspired me to submit it as it's own top level post on HN.