YouTube
(launched in 2005)
official website; TV Tropes; Wikipedia

YouTube is something that I, and probably most people, think of as a video-sharing site to which anyone can upload content. It has been used, over the years, for all kinds of things, much of it not intended to be professional, or serialized, or anything. But it's also been used by many people who created actual series that they wanted to make available online, generally for free. And in 2014, YouTube itself started a subscription service called "Music Key." The service was rebranded as "YouTube Red" in 2015, now featuring content other than just music. In 2018 it was split into "YouTube Premium" and "YouTube Music." Personally, I've never subscribed to any YouTube service, and I'm not sure if I ever will. At this point, I'm not aware of any premium content that I have a major desire to watch, but there probably are some shows on the service that I'd like if I did check them out. For now, though, I still think of YouTube as a good site to find all kinds of video content for free, including countless independently made webseries (and web channels) that use YouTube as a distribution platform. So it's that kind of thing that I'll list here. (If I ever do subscribe to the premium service, I'll have a subheading on this page for such shows.)

Channels on YouTube for which I have pages include:

ADHD
ALTER
Anna Akana
Cartoon Hangover
Christy Carlson Romano
Council of Geeks
Crypt TV
DC Kids
DC Super Hero Girls
Disney
DreamWorks TV / Peacock Kids
DUST
Geek & Sundry
MsMojo / WatchMojo
React
RocketJump
Underlined

Series I've seen on YouTube that either aren't on specific channels or I simply haven't made (and don't intend to make) pages for their channels include:

Adult Wednesday Addams (no longer on YouTube)
American Girl short films
Ask a Ninja
Bloody Cuts
Childrin R Skary
City Girl
Dragon Age: Redemption (no longer on YouTube)
Grumpy Princess Show
The Guild
Halloween Night (originally on ShockTillYouDrop.com)
Justice League: Gods and Monsters Chronicles
Kids Try
Last Week Tonight (webisodes)
The Legend of Neil (originally on Atom.com)
Mina Murray's Journal
The Moonshine Jesus Show
My Little Pony: Equestria Girls (webisodes)
My Little Pony: Friendship Is Magic (webisodes)
Newsbroke
People vs Food
Pop Culture Detective Agency
Post-Apocalypto
Princess Rap Battle
Riley Rewind
Scary Endings
SciShow (also SciShow Kids, SciShow Psych, SciShow Space)
A Tell Tale Vlog
Tiny Feminists
Vigilant
Violet
Witching Season Films