Amazon Video aka Prime Video (see also Amazon Studios)
(launched as Amazon Unbox in 2006; renamed Amazon Video on Demand in 2008; rebranded as Amazon Instant Video in 2011; renamed Amazon Video in 2015; renamed Prime Video in 2018 I think)
IMDb; official website; TV Tropes; Wikipedia

The company Amazon.com was founded in 1994, and soon became a major online bookstore, as well as selling increasingly more kinds of things. Amazon Unbox started in 2006 as a place to rent or buy video online. Amazon Studios started in 2010, as a competitor to Netflix and Hulu, though they had begun co-producing films in 2008. (The first Amazon Original film was in 2015.) They began producing original TV series (or more accurately, webseries) in 2013. I was aware of Amazon making original shows at that time, though I didn't really get a chance to watch anything regularly until fall of 2017. I should also say that some shows which are labeled on the site as Prime Originals, are actually acquisitions. Anyway, I'll list any webseries I've watched on Amazon that the site calls Prime Originals, I guess. (As long as Amazon is the original distributor in America.)

stuff I've seen:

Amazon Originals
An American Girl Story
Betas
Cinderella (movie)
The Dangerous Book for Boys (I watched all six episodes, but didn't write a review)
Good Omens
Gortimer Gibbon's Life on Normal Street
Hotel Transylvania: Transformania (movie)
Invincible
Just Add Magic
Just Add Magic: Mystery City
The Kacey Musgraves Christmas Show (special)
The Legend of Vox Machina
Lore
Lost in Oz
The Man in the High Castle
The Marvelous Mrs. Maisel
Niko and the Sword of Light
Sigmund and the Sea Monsters
Tales from the Loop
The Tick
Totally Killer (movie)
Troop Zero (movie)
Undone
Z: The Beginning of Everything

Acquired Series
Ordeal by Innocence (BBC 1, UK)
Philip K. Dick's Electric Dreams (Channel 4, UK)
Picnic at Hanging Rock (Showcase, Australia; I watched 3 of the 6 episodes before deciding not to continue)
The Pink Five Saga
Ronja, the Robber's Daughter (NHK, Japan)
Totally Real Movies Coming Soon (a collection of shorts from Funny or Die)

stuff I want to see:

Britannia
Carnival Row
Fleabag (A.V. Club)
Forever
Goliath (A.V. Club)
Good Girls Revolt
Hanna
Homecoming (A.V. Club)
Hunters
Modern Love
Mr. & Mrs. Smith
Small Axe (BBC One)
Sneaky Pete (A.V. Club)
Solos
Upload
The Widow
See also web films I want to watch