Alter: Firefox extension that redirects YouTube, Twitter and Reddit to Invidious, Nitter and Teddit respectively. Source / Firefox
SponsorBlock: A crowd-sourced extension to skip sponsorships. Support invidious instances if enabled in the options. Source / Firefox / Chrome
Invidition: Firefox extension that redirects YouTube links and embeds to their Invidious counterpart without any call to YouTube. Source (deprecated).
Alternate Tube Redirector: Extension to automatically open YouTube Videos on alternate sites like Invidious or Hooktube. Source
Invidious Copy URL: Adds context menu options on Invidious to copy shortened YouTube URL at current time or not (Requires using developer mode in Chrome or a developer version of Firefox).
YouTube to Invidious: Scans current page for YouTube embeds and replace with Invidious.
No-Youtube: Userscript to replace youtube links to invidio.us.
Invidious Audio Mode: Adds a headphone icon next to all video links. Handy for those that just want to stream audio and not video.
FYTE: Replace all YouTube embeds on a page with the video's thumbnail. Click on the thumbnail to play the video. Invidious is supported in the Options panel.
Extensions that integrate Invidious into other programs and apps
UntrackMe: Android app to rewrite YouTube links to Invidious. Can optionally play videos in the app as well.