thx

If you get annoyed at the recent rise of "But first, I have to talk about our sponsor for the day" interruptions in your YouTube videos, this add-on is an actual godsend.
Just like you should use µBlock Origin for the web in general, you should use SponsorBlock for YouTube specifically.
Just remember to support your favorite creators in other ways, whenever possible.

a good youtube extension to save time to watch even more youtube videos.

I'm glad to have found this by accident. Since installing the addon last week I have been adding skip segments to lots of videos myself and it's really satisfying to do so. Seeing the counter that shows you how much life time of other people you have saved increase with each passing day is really fun.

Does what it says it does. Obviously with smaller Youtubers the coverage is not as good, but I imagine it will get better as the app grows. Highly recommended, it circumvents all the annoying in-video ads and other superfluous stuff.