The runaway success of The Best Page in the Universe back in the late 90s spawned a whole genre of websites that follow the pattern of, "I run an angry blog where I write offensive things". Many even went as far as to title their pages akin the original with examples like, "The Second Best Page in the Universe", or even, "The Worst Page in the Universe". Most imitators unfortunately ignored the original website's satirical edge and instead just aimed for shock value.
In that vein, The Agony Booth re-tailored their site in the early tens to emulate That Guy with the Glasses, supplementing their old text reviews with "prop comic" reviewers and videos. Most of these were received with lukewarm responses at best. Unfortunately the long-form recaps that had been the site's signature and unique appeal were completely abandoned to make room. The majority of the new critics were Spoony Experiment knock-offs who used to advertise on his forum and on YouTube before being picked up. A similar website, Reviewtopia, was lambasted for trying to copy the success of TGWTG, and most of those critics were absorbed into the TGWTG mothership within a few months. The "success" of The Blockbuster Buster and Oancitizen came about because of this.
Stories based on The Slender Man Mythos have become increasingly popular due to the success of Just Another Fool (blog) and Marble Hornets (video series). Some recent cases have abandoned the idea of the characters going in blind and actually mention the existing blogs, with the protagonists assuming (wrongly) that they're just an urban legend.
Reviews were commonplace, but The Angry Video Game Nerd and The Nostalgia Critic started the trend of funny ones. They have since achieved online notoriety and several others attempting a style similar to them (i.e. that of a foul-mouthed and pissed off reviewer of old crappy games and/or films). It's worth noting that the Angry Video Game Nerd himself was far from being the first to do this. Seanbaby had done this years before (and was likely an inspiration), and he may not have been the first himself.
Unfortunately, because of his rabid fanbase, those who actually try something new with the idea or take an alternate approach are often ignored or flamed for being "rip-offs". Even people who review ANYTHING made in the 80s such as cartoons and comics (just look at The Nostalgia Critic) get branded as rip-offs. Many hold the opinion that some of his genuine ripoffs are better than Mr. Rolfe himself is anyway, so to some this is a moot point.
Speaking of the Critic, the fact that he got branded as a rip-off was the reason why his feud with the Nerd started in the first place (see Nerd Rant 1 for proof). Ironically, a lot of contributors on That Guy with the Glasses have copied the Critic's style, down to even the camera standpoints and way of executing gags.
Oancitizen has often been accused of being a rip-off of The Cinema Snob, despite the fact that Oancitizen reviews arthouse films without being snobbish while the Cinema Snob reviews exploitation movies while pretending to be a snobbish critic.
Yu-Gi-Oh! The Abridged Series has had many knockoffs. The creator has even done voices for some of them, as well as lampshading it in his own series, by turning the "Multiply" spell card (which makes copies of Kuriboh) into "LittleKuriboh Impersonators". Yami activates it in his duel against Pegasus.
After Franchesca Ramsey made the Shit White Girls Say...to Black Girls video, many others followed up with videos following a similar format of a marginalized person performing as a privileged person saying ignorant and bigoted remarks to a person of their marginalization.
That video was in turn inspired by the video "Shit Girls Say", based on a Twitter page, which also had various imitators with black girls, Spanish girls, etc.
After Chuggaaconroy put Let's Play-ing into the mainstream with his wacky Manchild antics, loud moments, and Epic Fails, a spat of other YouTubers trying to find the same success imitating this style without any of the approachable personality or passion Chuggaa puts into his videos. If anything, this following also led to Retsupurae poking fun at these pale imitations. Ironically, Chuggaa himself has stated that his inspired by ProtonJon.
Ben Drowned. It is the most famous gaming Creepypasta that was filled with horror, great effects, and a great story. It was so popular, that we got many other gaming horror stories like Sonic Exe, Megaman's Ladder, I Hate You, and many more stories that don't even come close to capturing the atmosphere nor the creep factor that Ben Drowned had. Blank cartridges or discs with handwritten titles on them (which were free, of course. Even at brick and mortar stores.), friends who mysteriously disappeared in the context of the story, spooky save file shenanigans, and simplistic romhack footage on YouTube are some of the elements that label them as derivative.
Markiplier seems to follow the PewDiePie formula of ADVERTISING VIDEOS IN ALL CAPS, photoshopped title cards, and OTT panicking at survival horror games. Over time, he's even adopted Pewd's charity work, haircut and five o'clock stubble, as well. Amazing.
CinemaSins has seen dozens of imitators on YouTube, from single videos to long-running channels reviewing different kinds of visual media. Two of the earliest examples — Game Care Network and Dartigan, both video game reviewers — now have their own pages.
The runaway success of both Game Theory and V Sauce have inspired many other youtube channels to create their own scientific analysis and Wild Mass Guessing of pop culture works be it video games, movies, TV shows, or even internet stars. Some like the Super Carlin Brothers have taken it even further combining the pop-culture hypothesizing of Game Theory with the vlog format of the VlogBrothers.
While doing Alex Reads Twilight, Alex repeatedly insisted he would not be doing the rest of the Twilight series. This spawned a host of imitators that picked up with the second book in the series, New Moon, copying his general man-in-front-of-a-camera-summarises-and-snarkes-about-a-book style.