The There she is!! series of videos at http://www.sambakza.net/ feature Doki and Nabi, a rabbit who falls in love with a cat. The first two videos are cute and funny, showing Doki chasing Nabi despite his arguing that they are different species, then him getting her a birthday cake. The third and fourth videos show how society is REALLY against this pairing, with the two of them getting injured, property destroyed, pet dying, etc. Not so fun anymore, huh?
Jib Jab, originally, light in tone, went from gently mocking Bush and Kerry in 2004 to mocking McCain and Obama straight up in 2008, along with the broken promises made by presidents in election day that never come to light when they are actually in office. The "Year in Reviews" went from hoping it'll get better to, most recently, the 2008 Year in Review summing up everything that's bad, and adding "wars and famine" to the end.
Super Smash, the gritty reimagining of Super Mario Bros makes itself to be a rather awesome flash.
Singer: There's new demographics when nobody asked for it!
The third season of Gotham Girls is much darker and realistic than the last 2. In fact, many DCAU fans consider this to be the ONLY canon season.
Red vs. Blue: The Blood Gulch Chroniclesnote (Seasons 1 - 5) is a Random Events Comedy, that just gets more insane as it progresses. From Reconstructionnote (Season 6) onwards however, plots are a lot more focused, backstory is elaborated on, justification is provided for The Blood Gulch Chronicles' Random Events Plot (Without even a single Retcon), action scenes are more frequent and intense, and just about every character gets rounded out (Some are surprisingly complex). Revelation and The Project Freelancer Saganote (Seasons 8 - 10) in particular have some genuinely great character drama.
Season 11 looked like it was a return to the comedy of the original format, but halfway through gets as dark as the series has ever been.
The latest seasons The Chorus Civil War takes the same format. At first it started the same way as the earlier season, and things get darker as the season progresses.
Although already dark, Episode 10 of the webseries Salad Fingers could very well be darker than the previous episodes. The episode in question is entitled "Birthday," and it starts with our protagonist Salad Fingers about to throw a party for Hubert Cumberdale, his finger puppet. He then goes to what he refers to as Milford Cubicle's corpse (which was still hanging from the rack where SF left it in the episode "Nettles") and asks it to take a hot bath and prepare for Hubert's party. SF then gets a knock at the door, and he sees a long pole that's extending to the sky. Worried about Milford, Salad Fingers leaves his horse Horace in charge of the house until he got back. He walks out into the forests and goes through some trash and finds a puppet that he calls Dr. Papanak. Dr. Papanak then examines him and unexpectedly bites him. Salad Fingers then throws him over to a horse that resembled Horace, and the puppet began to cut into the side of the horse. Salad Fingers witnesses this, and assures the horse that he knew it hurt, but he should try to hold still and let the puppet consume its blood. Salad Fingers then takes a nap and when he wakes up, he seemed older stating that he was asleep for probably 'Six Mondays.' He then goes back to his home to discover that more horses were at his house and that the flesh on Milford Cubicle's skeleton had been cleaned off. Salad Fingers is enraged and casts the horses out of his house. He then goes outside and sees the pole again and asks it if it was the ringleader behind Cubicle's fleshless corpse. However, it turns out that five Salad Fingers-like beings had eaten the flesh off of the corpse and were seated at a table. Salad Fingers then goes to the table and sits down. The pole soon extended from the sky and a present was revealed to be at the top of it. Salad Fingers opens it and it turns out to be a hat made out of Cubicle's face. SF puts it on, and stated that he'll wear this hat until his death.
Mario Brothers is this to the Super Mario Bros. franchise. King Koopa is a completely serious and terrifying villain here, and deaths are treated with the seriousness death entails.