Huey Freeman of The Boondocks had become the straight man of Riley Freeman, Robert "Granddad" Freeman... and even Uncle Ruckus.
Carey Means, who plays Frylock on Aqua Teen Hunger Force, has the added privilege of playing the Straight Man to lines that usually don't even make sense spoken by anthropomorphic food items and stoned two-dimensional aliens.
That said, whenever Twilight goes off her nut Spike typically steps up to fill the role.
Additionally, Fluttershy and Applejack are usually the ones to play this to whatever character is serving as their comedic foil at the time, while it is more rare for Rainbow Dash, Rarity, and especially Pinkie Pie to do so. The character dynamics of "Hearth's Warming Eve" are built around the three "straight" characters playing idealized versions of themselves that are paired with the three "funny" characters, whose personality flaws are dialed Up to Eleven.
Cheerilee plays the comic foil to the Cutie Mark Crusaders in "Family Appreciation Day" and "Hearts and Hooves".
Ulrich of Code Lyoko often plays the Straight Man to Odd's antics.
Brian of Family Guy infamy started out as a simple foil for Peter and/or Stewie's wackyness. The evolution of Brian's character gives us an interesting case study in exactly what level of Character Development is appropriate for these types. If all they ever do episode after episode is set up other character's jokes and snark at their buffoonery, you run the risk of having a Flat Character. Going too far in the other direction, though, takes us into muchmoremalignantterritory, as giving too much exposure to a character whose main purpose is appearing more sensible and intelligent than others will eventually necessitate the writers revealing their own prejudices about just what they think an intelligent or sensible way of life is.
In The Simpsons, Lisa has the thankless task of playing foil to her entire town. In the early days she and Bart were a classic Straight Man and Wise Guy combo, and while this is still true, she often finds herself playing straight to almost everyone else in Springfield. Luckily, she is usually excellent at it. Occasionally, especially in later seasons, Lisa's more extreme personality traits will come to the fore and she gets to have someone else play off her for a change. Lisa was in danger of falling into the same trap as Brian (see above) in later seasons, as she began to turn into a Soapbox Sadie. In more recent years the writers have dialed her back to her earlier, more sensible persona; she hardly ever mentions politics anymore, and she has once again become the straight man.
Stan Marsh from is the Straight Man to the WHOLE town in later seasons. While Stan is probably the "straightest" character, others exist too.
Kyle is Straight Man almost as much as Stan, especially compared to Cartman.
Sharon will often play Straight Woman to her less sane husband Randy.
And in general, kids are often the Straight Boys and Girls to the town's idiot adults, though at times the Mayor will do this too (despite at other times being an Authority in Name Only or be just as stupid as them). The premise of the entire show basically boils down to the idea that all kids are straight men to adults: they stand on the sidelines and comment with bewilderment on the insanity that is the adult world.
On Invader Zim, Zim's insanity is usually contrasted to Dib, though his base's supercomputer or even GIR can play this role in a pinch. In general, though, everyone will play this to everyone else at some time or another, because they're all at least slightly insane.
When Dib needs to be the insane one when trying to warn everyone about Zim, his sister Gaz provides the foil.
Both of them regularly foil their mad scientist father whenever he comes up with a new invention.
Ever since soon after he was created, it has been customary to pair Daffy Duck with a Straight Man — usually Porky Pig, who has been the go-to straight man for Looney Tunes ever since his creation years earlier — but later on Bugs Bunny occasionally played straight man to his then more abrasive personality.
Much like his Looney Tunes predecessor, Hamton Pig in Tiny Toon Adventures often plays this role. Babs even invokes the trope in the first episode. He's often straight man, to Daffy's counterpart Plucky, though one notable plot played Plucky as suffering straight man to Hamton's ridiculously nuclear family.
Perry's voicelessness puts him in the straight man position to Doofenshmirtz (and, to a lesser extent, Major Monogram).
And, of course, Linda, despite her own goofy moments, plays the perfect straight woman to Candance's obsessive craziness. Of course, it's the straight woman attitude that is causing a lot of that craziness...