Dr. Cameron has big eyes and looks like a sad little kitten quite often, usually when somebody points out she takes things way too personally and that she should act more like a doctor, not a friend, when dealing with patients.
Dr. Wilson has big chocolate eyes. Cynical House MD sometimes laughs at him for it.
Dr. House isn't exactly adverse to looking like a kicked puppy when someone hurts him in one of his weak spots, either. His initial reaction to his cane snapping in half in "Safe" and his emotional moments with Stacy are good examples.
Cuddy: I'm here if you need me. House: ...I need you.
For a more deliberate (and hilarious) version of this trope, see "Deception", where House needs a lab technician to run twenty-nine tests on one cc of blood. She refuses. House compliments her eyes. She starts to say something sardonic and turns — House, chin on a nearby centrifuge, slight sad smile, eyebrows up, gives her a soulful blue look. She did the tests.
Lt. George from the fourth season of Blackadder can do a particularly heart-rending version. Hugh Laurie has a particular talent for looking like a kicked puppy.
The main character Scott McCall has the most goddamn adorable, big brown puppy dog eyes. Add to that his heartmelting smile, and you have to either be A) heartless, or B) Mama McCall to not succumb to it.
Kate Argent, the most heartless character on the show, cannot resist them and says,"You, with the adorable brown eyes, (drop your bike, you're staying for dinner)" and "... Because I really do love those adorable brown eyes." (Although once Kate finds out Scott's secret, well... if you saw Code Breaker, you'd know.)
Despite having more hazel-green eyes, Derek Hale is quite skilled at this.
Despite being a self-described shark and a Magnificent Bastard (at least initially), Nathan Petrelli has one of the most effective, soul-melting puppy-faced looks in the business (case in point: Into Asylum). ...Damn you, Adrian Pasdar.
Sylar manages to pull off this look on occasion, when he's not being completely psycho.
Peter, when he's trying to convince Nathan to go along with one of his ideas.
In one episode Q tries to seduce Janeway by making a puppy appear in her quarters. Janeway replies, "It's not going to work, Q." Q picks up the puppy and asks, "How can you resist that face?" then gives his best "dog eyes".
Star Trek: Deep Space Nine Major Kira has a pair of absolutely enormous, soulful brown eyes, and as such was also quite good at these, although she used them much more rarely.
We have Bertram Wilberforce Wooster from Jeeves and Wooster. He doesn't even really try, he's just like that. Actually, most of Hugh Laurie's characters are guilty of this. D'AWWW.◊
Lampshaded in the show premiere of Angel, where Doyle refers to Buffy as "Miss Young Puppy Eyes" when giving Angel's back story.
Giles was accused of this once by Jenny Calendar. And when he replies "I didn't mean to make 'dog-eyes' at you..." it has the same effect on her.
Buffy herself tries it on Giles, to get out of training and go to a party. He stoically resists.
Part of Willow's Woobie appeal is her ability to do this. Must be something about Alyson Hannigan. As Joss Whedon put it — "There is nothing more painful in the world than Aly when she makes her big eyes."
Angel himself. At least one review of the show placed at least part of the shows success on David Boreanaz's ability to always look like a slapped puppy.
Ted in "Crazy Eyes" (also known as "Swarley"), where he reveals he has a tailored look for impressing women: Head down, eyes looking up, hands in pocket, shoulders up, head slightly tilted, conveying shy vulnerability. He teaches is to Marshall who unfortunately doesn't quite get it right.
Robin has her look, but she doesn't use it that often. It goes like this: biting her lower lip, shyly looking away and thrusting her chest. It's a combination of sad eyes and Coy, Girlish Flirt Pose.
Michael Westen from Burn Notice. Accomplished spy, brilliant fighter, master of the puppy eyes. In the pilot episode he was talking with an old friend Lucy for some help (just being a burned spy would not be good for Lucy's reputation to help him). As she continued to say "No" he just kept staring at her, eventually making himself look like a puppy putting his paws on the table. Lucy eventually caved.
Djaq (Anjali Jay) and Much (Sam Troughton) from the BBC's Robin Hood were the undisputed masters of the Puppy Dog Eyes. All it took was for either one of them to look sad.
iCarly: Freddie cannot resist when Carly follows up a request, or asking for a favour with a pouting "Please, for me?"
Occasionally Richard Hammond. Might not be deliberate: his eyes are naturally large and he quite literally looks up to the very tall Jeremy Clarkson. Clarkson likes to lampshade it by pointing out Hammond's resemblance to "that cat in Shrek 2."
Clarkson gets in an epic puppy-dog mope of his own in the S12E6 news segment when he has lost his voice and both his fellow presenters and the audience are merciless.
Tim McGee on NCIS, more so in the first few seasons, was prone to looking like a lost little pup more than a federal agent. Fan fiction writers have picked up on this and several stories simple revolve around different characters receiving the puppy eyes.
Donna's eyes look like this when she's greatly sad and distressed, as seen in "The Fires of Pompeii" and "Planet of the Ood".
Annie in Community uses this often to manipulate the group. Jeff explains this look as her "Disney Face" when she uses it to try to keep the group from being mad at her when she does something that might have made them have to retake a class.
Kurt Hummel (Chris Colfer) can make grown men cry with a mere look.
Blaine Anderson (Darren Criss) has puppy eyes that can make you coo more than if you were watching bunnies and kittens cuddle.
And Emma Pillsbury (Jayma Mays), which led to a funny scene where a drunk Kurt confused her with Bambi.
The Inspector Lynley Mysteries: DS Barbara Havers, whose large, expressive sage green eyes are more than capable of this, and clue the audience in to her vulnerability when she's covering it up with a scathing remark. It is, in fact, largely because of her eyes that Barbara is such a sympathetic character; if her eyes didn't provide a window into just what a good reason she has to be defensive and prickly, she would come off as a complete bitch in early episodes.
The Wire: Ziggy. Take a look at any scenes where he's upset, but particularly scenes involving Frank, his neglectful and rather disdainful father. For all Ziggy's faults, the jailhouse scene dialogue coupled with his big sad browns is a reason, why some viewers finally warmed up to him.
Mash: Col. Potter pleads with Hawkeye and B.J. to make Maj. Burns feel like part of the gang, and the two consent to it. Hawkeye says "How can I resist those big Colonel eyes?"
The Woobieness of Niles Crane in Frasier at least partly owes itself to the masterful skills of David Hyde Pierce's huge, sad, brown eyes. He does pull off that look with great success (occasionally earning an exasperated "Don't look at me like that!" from Frasier).
Josh Lyman of The West Wing absolutely owned this trope, particularly the facial expressions that go along with the eyes. He was outmatched only by the even more heartbreaking puppy dog eyes of his assistant, Donna Moss. Josh once explicitly told Donna that he couldn't bear to tell her a particular piece of bad news because her sad eyes haunted him.
In Firefly, River Tam sometimes has these in her more vulnerable and traumatized moments. Her other look is usually very close to "crazy eyes".
In Emily Owens, M.D., a blind patient tries to convince Emily to treat his dog. She tries to resist, telling herself that all dogs have sad eyes and that she mustn't look, but she does look and she's in. She then gets her colleague and friend Will to help her. She talks about those sad eyes, and Will agrees — Emily's look did it for him.
In Sex and the City, Charlotte York uses her vulnerable look on her fiancÚ Tray when they pick up their china, and his obnoxious mother is with them and tries to have her way.