Aday With Bowser Jr. The series began with sloppy Flash videos before slowly improving in quality. Today, the videos are animated in crisp 3D animation.
Homestar Runner. This is lampshaded in the eponymous flashback in the Strong Bad Email flashback; it features crudely-animated art based on that in The Homestar Runner Enters the Strongest Man in the World Contest, a children's book made by the series' creator before their website was established. In the Email lady...ing, Strong Bad presents a clip parodying the crude animation style and personalities of the very, very early Homestar Runner short Marshmallow's Last Stand.
Fairy Foxes has gone under Art Evolution throughout its 6 episodes. The foxes started out as cute little chibis. However, their designs changed to an anthro look and stayed that way since episode 3.
And since episode 7 came out, StickFreeks (the creator of Fairy Foxes) has switched from MS Paint to Adobe Photoshop Elements 7.0.
Much like the Fairy Foxes example above, Wolf Dragons also started out different than it is today. Here are examples of the pilot episode: Before and after. Additionally, voices didn't appear in the first version until around episode 3, while in the better version, there were voices from the start.
Red vs. Blue had a unique form of evolution: through the first few seasons, the voice acting and video quality improved somewhat, from the low-quality early episodes to much better sound and video by Season 4 or 5. In addition to progress with equipment, though, the series gained significant upgrades with the release of each new Halo game, and by Revelation, also features extensive CGI thanks to Monty Oum joining the crew. It comes full circle with the release of remakes of the first 5 seasons, re-filmed in HD.
It was stated at one panel that as their sound equipment gradually became better, fans started to complain that the voices sounded "too good" and the team decided to work the audio a bit to maintain the low quality feel.
Nameless progressed from relatively crappy quality to quality rivaling that of some professionally-made cartoons in a few episodes.
The BIONICLE Flash episodes (which consisted of cutscenes from the Mata Nui On-Line Game and a short series of non-game animations) went through some degree of evolution:
At first, both the animations and the game blended rough 3D graphics with crude 2D Flash animation. Some of the movements had many individually rendered frames showing off bodyparts/machinery from slightly different angles. The characters had a lot of shading, and the backgrounds were immensely detailed, though some of the textures were low-quality jpeg images. Characters were also often Off-Model, since their sprites were based on bad quality photos of LEGO pieces and primitive 3D models. As the game progressed, the visuals changed to standard 2D imagery, with simplistic Clip-Art Animation. The characters only had a few basic angles and barely any shading that matched their environment, which also became less detailed but more stylized and "uniform". So when you went back to the first areas of the game, characters suddenly looked out of place.
The Kanohi Pakari, the type of mask worn in the game by Onua, Kapura, Hafu and Onepu. When Onua first appeared, his mask looked really close to the real piece. By the end of the game, he and all other Pakari-wearers were outfitted with a completely Off-Model-looking mask that barely resembled the original. This is due to the game designers confusing the Pakari with the Ruru (mask of Whenua and Taipu), and created an odd mix of the two. They even switched the masks of Hafu and Taipu, and incorrectly gave Kapura a Pakari. Faulty reference images and miscommunication with LEGO are likely to blame. Beginning with the second episode of the web animations, the animators thankfully settled on a new design that looked exactly like the Pakari.
An Akatsuki's Life. From its somewhat crudely animated and muffled-sounding beginnings, it eventually evolved into this. The latest episodes contain a handful high frame-rate shots that (aside from the obvious pixels) wouldn't look too out of place in an actual cartoon show.
One could say the art style for Neurotically Yours went through this, starting from very rough design and animation and it now a lot smoother.
The art in Happy Tree Friends became less stiff and more "cute" over time, mainly by making them overall a little rounder and cartoonish. The overall animation quality also improved. A more specific example is the appearance of Flippy's evil Split Personality. In his very first episode, he looked exactly like normal Flippy. After that he gained greenish-yellow eyes, Creepy Shadowed Undereyes, and crooked teeth. This would remain for a few episode before the crooked teeth were replaced by fangs in late season 2, finalising the design.
Ashley in The Crazy Kids of Grade 5 has went through a character redesign. Compare this ◊ to this ◊.
Cow of the Wild has gone through this over three years. In the first episodes, the feet were large in comparison to the rest of the body, several wolves had a lot of facial hair (i.e. Alari's bangs), eyes were one color with white highlights, and cat characters were frequently mistaken for wolves, or vice versa. In the more recent episodes, the feet have gotten a bit smaller, the eyes are multicolored, and there's a distinct difference in the basic character design of different species. The lineart and shading have also become much more refined.
DSBT InsaniT: While it still keeps its simplistic style, there is a BIG jump in quality from episode 1 to episode 2.
Starting from Episode 4, the characters became more expressive and gained Four-Fingered Hands, both of which had been implimented from the start in Dreamscape.
Starting from Episode 6, the art style for the characters gets a complete overhaul by being designed in Adobe Illustrator. The same goes for Dreamscape.
The music videos for Savlonic, a fictional band created by "Weebl" Picking, have undergone this—the third and fourth videos, created by another animator, are far more advanced graphically and no longer depict the characters in borderline Super-Deformed style.
Turnabout Storm's art and animation quality saw a few improvements after the fist two episodes for different reasons:
Ace Attorney characters, who use their in-game sprites, went from the pixely sprites of the Game Boy Advance and Nintendo DS releases to the much smoother artwork of the iPad's HD re-release whenever they are compatible. Besides that, the dialogue box's font was changed to the same one used in Ace Attorney.
While the Ducktales characters still look as crappy as they did in the first Ducktalez, Vegeta has seen an incredible improvement between Episodes 3 (fairly well animated for a Flash cartoon in that age) and Episode 7 (professional-quality CG animation).
GEOWeasel goes from very Flash-influenced animation with same-width pencil outlines to a very messy but more varied style in episode 9 that eventually evens out through the rest of the series.
Many Garry's Mod users tend to do this as time goes by and they learn the craft more. Notable examples include RubberFruit, Dr. Face, and Eltorro64Rus, all of whom started off with standard sketchy animation and eventually worked their way up to something you'd expect from a big budget stop-motion film.
RWBY undergoes a pretty steady evolution due to both the growing crew and tightening schedule.
The original trailers, primarily made by series creator Monty Oum, featured stark, stylized environments and enjoyed extremely fluid animation due to being released months apart.
Season 1 proper switched to more generic, somewhat tacky backgrounds and generally rougher animation.
Season 2 began utilizing a darker and more futuristic look for the backgrounds and featured generally better animation due to new and improved character models.
Season 3, the first produced following Oum's premature passing, uses the same models but with higher polygon counts.
Season 4 features a completely different style due to switching art programs. Every major character received a new model.
Deliberately invoked in season 7 of Sonic for Hire. The end of the previous season had Sonic resetting a Sega Genesis, and essentially, existence. The only thing that remains of the characters is a couple of moving pixels. Halfway through the episode, the characters start to rebuild their life, and the artstyle resembles an Atari 2600 game. Come episode 2, and the artstyle is back to normal. It did take a whole week for this to happen, though.