Vimeo
(launched 2004)
IMDb; official website; Wikipedia

This is something I don't think I ever thought of as a platform for original webseries, until the first season of Con Man was announced. I still haven't seen that, and it's the only series I'm sure originated on Vimeo, but I do hope to see it, someday. Meanwhile, I do find Vimeo useful for watching all sorts of things that originated elsewhere, especially short films. And... I guess I'll include some webseries here, even if I'm not actually sure whether or not Vimeo was where they debuted. (I mean, I don't always know whether YouTube is the first place something streamed, either, so why not?)

stuff I've seen:
Downward Dog
The Pink Five Saga (available on Vimeo briefly, before moving to Amazon, which is where I saw it)