(example: youtube, vimeo, soundcloud, ceskatelevize.cz, Google Drive…)

The venerable youtube-dl. (supported sites)

Always try this first, then look around for alternatives.

Make sure you're up to date though! (youtube-dl -U) YouTube likes to play a cat & mouse game, meaning youtube-dl breaks periodically… If a download errors out, there's a 95% chance you're just using an outdated version and it has been already fixed upstream (they make a new release every week or so).

(example: imgur, flickr, 4chan, tumblr, twitter…)
Analoguous to youtube-dl, there's gallery-dl (supported sites)

(example: Google Arts & Culture, all sorts of hi-res scans…)
For all zoomable images, try https://dezoomify.ophir.dev/dezoomify/dezoomify.html (also has a standalone version)

• https://addons.mozilla.org/en-US/firefox/addon/hls-stream-detector/
  • For grabbing live streams or even obscure video sites. Usually an intermediary step to get an URL to put into ffmpeg or youtube-dl.
• https://www.downloadhelper.net/
  • Good for sites that require authentication or are otherwise awkward to rip via youtube-dl.
• https://www.downthemall.org/
  • Classic addon for grabbing webpages with dozens of direct links, or obscure image galleries.