30 Rock

I'm sure 30 Rock had numerous webisodes on NBC.com at some point (Kenneth the Web Page, Ask Tina, Dear Tracy Jordan, Frank vs. Lutz, Livin' XL with Grizz & Dotcom, Jack Donaghy: Executive Superhero; possibly other stuff, I dunno). I don't remember if I ever watched any of that stuff, but I certainly couldn't have watched much of it. Which is a shame. Someday I really should look on YouTube or something and see what I can find.

Community
TV Tropes; Wikia; Wikipedia

Community had a number of webisodes (whether one-shots or miniseries) but the only one I recall watching is Abed's Master Key. I really should try to watch the other ones, someday.

Eureka: Hide and Seek (originally on scifi.com)
IMDb; Wikipedia; YouTube

Based on the series Eureka. This was a series of 8 episodes that aired online in July 2006, during the show's first season. I have no recollection of having watched it, but since I know it exists, I thought I'd mention it here. Maybe someday I will get around to watching it.

Farscape

The folks behind Farscape have been promising us new webisodes since 2007, but so far I don't believe it's happened. For awhile there were some minisodes on YouTube, but those were just episodes of the TV series that had been recut into shortened forms about 6 to 9 minutes per minisode. (I didn't watch them, and as far as I know, they're not there anymore.) So Scapers must continue to wait for something new....

Heroes Reborn: Damen Peak (originally on nbc.com)
Heroes Wiki; Wikipedia

This is a series of three webisodes released between November 19, 2015 and January 7, 2016, several months after the TV series Heroes Reborn ended. However, I didn't learn of its existence until 2019, probably because I never got a chance to finish watching the TV series. (Though it's also possible I wouldn't have ever been aware of it before now even if I had seen the whole series.) In any event, I suppose I should try to find it sometime, if I ever get around to finishing the TV show.

My Little Pony: Friendship is Magic, on YouTube
There have been several series of webisodes for both the regular "Friendship Is Magic" series and the Equestria Girls movies. I've seen a couple of the latter types, but there's another series of shorts called Better Together, which I still need to watch. (And when I do, I might put it under "animated webseries" instead of "webisodes," I'm not sure yet.) As for the ponies, one of these days I need to watch My Little Pony Established 1983, and some random shorts. (There are a few other series of MLP and MLP:EG shorts that I haven't seen and probably won't bother with.)

Psych Webisodes (originally on usanetwork.com)
IMDb; TV.com

Based on the series Psych. This was a series of six webisodes that aired USA Network's website in 2006, during the first season, I think. I have no recollection of whether or not I saw them, but they're not on the website anymore. I could probably find them on YouTube or something, so maybe I'll watch them someday.

Psych: Flashback to the Teen Years
IMDb; TV.com

I guess this was a series of three webisodes from 2009, during the show's fourth season. It was about Shawn and Gus as teenagers, but I don't remember if I saw it or not.

Smallville (originally on AOL / Sprint phones / thewb.com / cwtv.com)
Wikia

Smallville had several miniseries of webisodes (or mobisodes), the only one of which I've seen is The Oliver Queen Chronicles, during season 6. I'd like to check out the others someday. They include The Chloe Chronicles (during seasons 2 & 3), The Vengeance Chronicles (season 5), and Kara and the Chronicles of Krypton (season 7).

True Blood: Drop of True Blood (originally on hbo.com)
IMDb; Wikia; Wikipedia

These webisodes were released between the second and third seasons of True Blood. But so far I've only watched the first season of that show, and I'm not sure when I'll get around to watching the second. Nor do I know if I'll even be able to find the webisodes online.