I’m thinking along the lines of the Flintstones being used for the Fruity Pebbles brand. Not characters that were made up for the campaign, like Cap’n Crunch. Any other examples? Apologies if this has already been done.
Where do you want to start? Commercials using cartoon characters are all over the place.
Bugs Bunny, Daffy Duck, Wile E. Coyote, and the Roadrunner appeared in an Aflac commercial.
Elmer Fudd, Foghorn Leghorn, and the Tasmanian Devil have all been in Geico commercials.
Bart Simpson has done Butterfinger commercials.
Stewie and Brian Griffin for “Hweat” Thins. (Stewie’s pronunciation.)
The Pink Panther for Corning insulation.
I’m sure Dopers can come up with hundreds of other examples.
Anywhere. Game on.
Snoopy and Woodstock for that insurance company (Metropolitan Life).
Rocky and Bullwinkle for GEICO, General Mills, Taco Bell.
Well, Fred and Barney knew A Winston Tastes Good, Like a Cigarette Should.
But everyone shilled for a specific sponsor back then.
Captain America, in a PSA for the Department of Energy.
The Peanuts characters also did commercials for Dolly Madison cakes.
I remember a Visa (“Priceless!”) commercial from about six years ago featuring a string of cartoon characters, including Olive Oyl modeling a WonderBra. Also, an ad for either American Express or Exxon (they were at a gas pump) with an animated Superman and a live-action Lois Lane.
Popeye’s Chicken briefly licensed Popeye for commercials back in the 90s.
Mr. and Mrs. Potato Head are currently in an ad for Lay’s Potato Chips.
Davy and Goliath for Mountain Dew. “We got hosed.”
Gumby and Pokey for Honda.
Despite the fact that Popeyes was named after a movie character, They used to have the cartoon characters all over the restaurant.
MetLife commercial with dozens, maybe hundreds of TV/Movie cartoon characters.
For a long time*, Dennis the Menace was associated with Dairy Queen.
*Actually, from 1971 through 2002. While researching this, I found the Wikipedia article that gives a name to the kind of thing this thread is about: Spokestoon.
Do cereals named after existing cartoon characters count? If so, there’s Strawberry Shortcake, ad infinitum.
Disney produced a lot of commercials using its animated characters in the 1950s. See here for the story, and here for an example. (By the way, Peter Pan peanut butter was not, as I had thought, a commercial spin-off of the Disney movie; it was in existence long before.)
They’ve always been a little cagey about where they got the name from; Here’s the Wikipedia account:
My take is that they infringed on the copyrighted character within microns of it being actionable, and then licensed said character to a limited degree, for a limited time.
More appropriately than fried chicken, there is a Popeye Brand spinach.
I hope it’s smothered in olive oil! 
Picking cartoon characters makes this too easy.
One thing that came to mind was Steven Van Zandt’s commercial for World of Warcraft. The spot doesn’t actually identify him as Silvio Dante, his character from the Sopranos, but there’s no doubt that’s who he’s playing.
I recall at least one year when Popeyes offered a discount on 17 January to celebrate some anniversary of the character’s first appearance.
The Flintstone appeared in commercials for cigarettes (winstons). These ran during the show itself – proof positive that the cartoons were meant for adults, as well.
For that matter, Hannah Barbera ran commercials for Kellogg’s cereals, not only involving their own characters, but animating the cereal mascots as well (the rooster from Corn Flakes, Sammy the Seal from Sugar Smacks, Tony the Tiger from Frosted Flakes)
Going way back, did you know that Skippy peanut butter is named after a comic strip character? The company denies it, but there is persuasive evidence that the tie-in is deliberate. The creator of the strip got no compensation, and he and his heirs have been fighting the use of the name: