In the English translation of Black Jack, Pinoko talks like this.
Used with annoying frequency by the titular character in the official English translation of the Chi's Sweet Home manga. Occasionally, they'll even toss Ws into the middle of words that HAVE NO CORRESPONDING L OR R SOUND.
To be fair to the translators, they admitted they would rather not have done it (they know it's annoying) but the people they licensed it from made them do it. That's because in the original, Chi's speech was apparently modeled after Tweety Bird.
In the original Japanese version, "d" sound is replaced with "r", "ru"s are now just "u"s, and "shi" becomes "chi". For example : "desu" becomes "resu" and "miruku" turns into "miuku".
Cebolinha (Jimmy Five) from Brazilian comic Monica's Gang, though exchanging just "R" - and not the whole time (in the original, he exchanges "R" for "L" - except when the words end with that letter, e.g. the totality of Portuguese verbs). In English translations, the speech impediment is the normal Elmer Fudd thing with the W.
Impressive Clergyman: Mawidge. Mawidge is what bwings us togevvah today. Mawidge, that bwessed awangement, that dweam wiffin a dweam. And wuv, twue wuv, wiw fowwow you fowevah and evah? So tweasuwe youw wove? Have you the wing?
"I AM DE DWEAD PIWATE WOBERTS!!" — of course, this was actor induced; not character-related.
Pontius Pilate in Monty Python's Life of Brian: "I've had enough of this wowdy webel sniggewing behaviow. Silence! You call yourself Pwaetowian guards?"
And Jeremy Irons' version of Havelock Vetinari does it, too. Not that it detracts from his intimidation factor.
Oh, you wascawwy Wincewind. It's the awena foah you!
Hooray For Wodney Wat is a children's book where Rodney's speech impediment is used to Rodney's advantage against a loud and annoying new classmate, to the delight of the west of the class. This was followed up with Wodney Wat's Wobot, in which the annoying new classmate weturned, but was again outwitted.
In Tickle Amongst the Cornstalks by Bob Bishop, lady Charlotte has this impediment (an unusual case of a Love Interest afflicted with this).
In the Flashman series, Lord Cardigan, a historical figure has this accent, representing the British Upper-Class Twit version. For instance, whenever he says the protagonist's name,, it's spelled phonetically as "Fwashman".
Like much of Flashman, this is Truth in Television (if not necessarily for Cardigan himself). Many aristocratic British officers, especially in the cavalry, deliberately affected such an accent to set themselves apart from lower class officers.
In the third installment of the Howl's Moving Castle series, "The House of Many Ways", Howl disguises himself as a young boy named Twinkle who talks in an overbearing lisp. Needless to say, it drives Sophie mad.
A minor character in the first Foundation book by Isaac Asimov talks this way.
The cavalry hussar, Denisov, from Tolstoy's War and Peace has this quirk. English translations tend to pweserve it.
Silas Heap in Septimus Heap is mentioned to sometimes invoke this when he's telling stories.
Richard "Clever Dick" Cleaver, an Evil Genius in Kim Newman's Diogenes Club story "Cold Snap", suffers severely from this problem. As Richard Jeperson, one of the Club's greatest special agents, thinks to himself: "How cruel was it to give a speech impediment a technical name sufferers couldn't properly pronounce?"
Barbara Walters. And, of course, mentioning her means that you have to mention Gilda Radner's Saturday Night Live parody, Baba Wawa. One Baba Wawa sketch is a parody of My Fair Lady. By the end of it she's not only not cured but has passed her condition on to Henry Higgins (Christopher Lee!).
In an episode of The West Wing, C.J. had an emergency root canal that caused her to (temporarily) talk like this.
Angel Batista of Dexter has a bit of one of these due to his actor's very thick Cuban accent.
"I'm wetiwing to open a wesauwant."
Rick Pratt from The Young Ones has difficulty with "r" but not "l" (a childhood problem of his actor, Rik Mayall) and it only serves to highlight what a petulant whiny prick he is. It is particularly noticeable in the theme tune when he says "Shouldn't be afwaid."
Used again with a live radio drama Frasier was direting. Roz had to have emergency dental surgery that day, and arrived on set with an impediment. "I can't bewieve that one of our guests could be a ... [resigned] mubable muhberber."
The TV show based on Honey, I Shrunk the Kids featured Fwuffy, a robotic teddy bear that is infected with a computer virus. Fwuffy plans to take over the world. His use of this trope even carries over into his spelling. When Nick has to guess his password, it ends up being destwoy.
From the Doctor Who serial The Twin Dilemma, Romulus and Remus, known to the fans as Womulus and Wemus.
Most English songs from 1980's German singer Nena, in particular "99 Red Balloons": "to wuwwy, wuwwy, supah scuwwy, caw the twoops out in a huwwy..."
Lady Gaga's song "Bad Romance" features her, after singing most of the bridge in French, sing "I don't wanna be fwiends..." Strangely, this is the only time in that song she comes down with Elmer Fudd Syndrome. The very next repetition of the line is sung normally.
This, as well as the Nena example above, may be explained by the use of guttural R in German and French, which can sound like w to an English speaker.
Fallout 3 has a boy called Biwwy (Billy). He speaks like this. He offers to sell you his Waser Wifle. When you buy the Wifle, you realize that it's the gun's actual name! It's better then a "Laser Rifle" (Though Billy also calls those Waser Wifles).
He doesn't even notice his speech impediment:
The Lone Wanderer: "Stop talking like that. No one thinks it is cute."
Biwwy: "Stop tawking wike what? You'we weiwd."
Handel and Greta from the Spyro the Dragon series fit this trope, with one interesting twist: the fact Handel's English deteriorated to Elmer Fudd English from Ripto's Rage to Year of the Dragon: while Greta speaks Elmer Fudd throughout both games, Handel actually speaks normal English in Ripto's Rage but is speaking Elmer Fudd in Year of the Dragon. Did he get younger, or did she just get him doing it?
Kala'ma in World of Warcraft is a troll hunter with an impediment caused by a nasty scar on his lip. Be vewwy quiet, he's hunting waptors. And is also hilarious. The PC can help him out by "pwacing waptor twaps."
Kala'ma: "Be caweful with the waptors, they can be wewy wewy smawt. Last week two hewd me down while a thiwd beat me with a stick."
Chaos Cultists from the original Dawn of War. This eventually gave birth to the fan-character Cultist-Chan. "Hwee hav captoored eet for kay-oss" indeed.
Homestar Runner, who admits "I have twouble with my aws" in a "Puppet Jam" session with They Might Be Giants. (But his L's are fine.) His counterparts, Homsar, 1-Up and The Homestar Runner don't suffer from this.
Catbug from Bravest Warriors, although this is mostly due to his voice actor being so young.
Lolcats: LO Lspeak, the language of the lolcats, and also the way most captions are written in.
Among other Looney Tunes characters, Tweety Bird talks with Rhotacism, most notably his signature line, "I tawt I taw a puddy tat." Translated, of course, is "I thought I saw a pussycat" – which one time, Sylvester did say at least once.
Franklin's sister Harriet on Franklin, though she eventually outgrows it.
In one third-season episode of The Boondocks, Lamilton Taeshawn likes to "smoke wit' cigawettes."
Gussie Mausenheimmuh on An American Tail. This leads to problems when she declares that they need to organize a rally. She's voiced by Madeline Khan, who reprised her Lily on Schtup voice from Blazing Saddles (mentioned above).
Eileen "The Birthday Girl" from WordGirl, who talks with a lisp in order for people to give her whatever she wants.
The Critic. Parodied with a Cousin Oliver who pretends to have an endearing speech impediment - and has copyrighted it for himself.
2D of Gorillaz has a mild case of this in his speaking voice, combined with a heavy Cockney accent. Here's a sample. Of course, it goes away completely when he's singing, due to being voiced by different people for singing and speaking.
Caillou's sister Rosie. Apparently, she outgrew it sometime after Caillou's Holiday Movie.
The Talespin episode "Waiders of the Wost Tweasure" plays with this. A character afflicted with this tells Baloo about the "wuby wings", which he naturally assumes means "ruby rings". It actually turns out to be a pair of ruby wings that inexplicably actually grant the wearer the abilityto fly.
Octus starts talking like this in Sym-Bionic Titan upon watching and mimicing a children's cartoon, until Lance tells him to stop.
Doctor Who writer and script editor Terrance Dicks.
Longtime Celtics broadcaster Bob Cousy, leading Bill Simmons to say "It's a good thing Cousy doesn't call Red Sox games where he'd have to pronounce Trot Nixon's name."
Incidentally, many English accents sound rather like this. This can be unintentionally humorous if a character has a Funetik Aksent (for instance, the Artful Dodger in Oliver Twist.)
Same for some East Coast American accents, e.g. Boston.
1930s actress Kay Francis had some trouble with her 'R's and L's and was known around the Paramount lot as "the wavishing Kay Fwancis"
Art critic John Berger.
US politician Barney Frank.
British politician Roy Jenkins
Novelist and biographer, Peter Akroyd.
British TV historian Lucy Worsley, whose official day job title is, unfortunately, "Curator of Royal Palaces".
Another British TV historian, Kate Williams.
Dutch people attempting to speak English tend to do this, because in Dutch, syllables can only start with a rolling "r". The English "r" does exist in the language, but only at the end of a syllable. Some southern Dutch/Flemish accents use more of a German-style "r" sound instead, making it much easier for the speakers to adapt to the English sounds.
Cantonese lacks the "r", so many Hongkongers can't pronounce it right. The common substiture is "w", while "l" and the occasional "n" may arise, usually depending on the word, and what is deemed to be closer in pronunciation.