Alternate Tooniverse

A subtrope of Alternate Universe where the AU is inhabited by cartoon characters. Exists alongside a universe like ours inhabited by flesh and blood human beings. Usually toons and humans are treated as separate species, if they interact. Humans may be fearful of Toons because of their different abilities. Note, however, that to whom, where, and when Toon Physics apply depends on the work, and frequently on the Rule of Funny.

Comes in two flavors:

Type I: Like our world, only with cartoon characters interacting with real live people.
Type II: An alternate (or parallel) universe exists alongside our world, and it is populated by cartoon characters. There is often travel between this world and one much like our own.

Examples of Alternate Tooniverse include:

Examples of Type I

Film

Who Framed Roger Rabbit?
Looney Tunes: Back in Action
The Rocky and Bullwinkle Movie
Last Action Hero. In Jack Slater's movie world, cartoons mix with human beings in the police station.

Live-Action TV

Played for laughs in Community, when Abed manages to convince Troy he's found a doorway to one of these outside Greendale Community College by painting an animated version of himself on a wall:

Troy: That's impossible!
Abed: Nothing's impossible in here! Animals can talk, your heart is shaped like a heart, and the smell of pie can make you float! You have to believe, Troy!
[Troy is just about to run headlong into the wall when:]
Abed: [Leaping out from behind a bin] Wait! You don't have to believe.
Troy: [Clearly heartbroken] I didn't!... I didn't... [He storms off]
Abed: I may have done some damage there.

Western Animation

Bonkers was a cartoon, but some people in the cartoon were people and some were toons. It was originally a vehicle for Roger Rabbit but ended up being an Expy named Bonkers. An unusual example of an animated Type I.

Examples of Type II

Comic Books

Captain Carrot and His Amazing Zoo Crew took place on an alternate earth (Earth-C), which Superman crossed over into once. They have their own alternate too, Earth C-Minus, where the events that Rodney Rabbit writes in comic books take place for real.
Howard the Duck comes from a Type II.

Film

Cool World: an alternate universe exists, populated by toons (or Doodles as they were called; the humans were called Noids).
Space Jam
Enchanted
The film version of The Phantom Tollbooth.
Rock-a-Doodle
Garfield Gets Real, where both the "real world" and Garfield's world are CG animated.

Live-Action TV

The Fringe episode "Brown Betty" MIGHT have featured such an alternate universe, as seen when story-world Nina Sharp communicates with story-world William Bell using the window device (though this may have been a choice to go for a retro-aesthetic).

Video Games

Toonstruck (an old quest video game) has this as its basic premise, with the protagonist, a cartoon animator, ending up in his toon world.