"Gotham's White Knight", District Attorney Harvey Dent in The Dark Knight is trying to help bring down the criminal empire in Gotham. Sadly, he ends up turning into the demented Two-Face after a horrific accident that disfigures half of his face.
In Ella Enchanted Prince Charmont gallantly saves Ella's life exactly three times, first from a speeding carriage, second from an ogre's boiling pot and then despite himself he has her back in the court battle.
At the end of Ever After when Prince Henry shows up to "rescue" Danielle from Pierri Le Pieu.
John Boorman's Excalibur takes this pretty literally with Lancelot. In his first scene, his armor is buffed almost to a mirror finish. Lancelot's not the only one, however; all the knights seem to wear highly polished - and sharp edged - armor everywhere including bed. Including when they are entertaining company of the feminine kind. Oddly the women don't seem to mind. Maybe it's a fetish.
Thomas Marshall in Ironclad probably fits the mold the most within the story, being a Templar knight who is a Chaste Hero constantly harassed by a lustful noblewoman and wearing white robes in contrast to the other knights. He agrees to defend the castle from the invaders and certainly displays the "I fight so you don't have to" attitude.
The eponymous Leopold of Kate & Leopold is a nobleman from 1876, swept into modern times, who believes that Kate requires a chaperone on her date with her boss so he offers to go with her to protect her from his obvious intentions. When she refuses, he tells her boss, "Some feel that to court a woman in one's employ is nothing more than a serpentine effort to transform a lady to a whore." Imagine the look on a purse-snatcher's face when Leo rides him down on horseback.
Leopold: I warn you scoundrel, I was trained at the King's Academy and schooled in weaponry by the palace guard. You stand no chance. When you run, I shall ride, when you stop, the steel of this strap shall be lodged in your brain. [bag snatcher throws down the bag and flees, onlookers applaud]
Balian of Ibelin in Kingdom of Heaven tries really hard to live up to this trope after being knighted by his father, which is specially difficult during The Crusades while surrounded by religious fanatics and cuttroat nobles. Unfortunately, his morals get in the way of political pragmatism such as when the king on his deathbed requests that he marries his sister (whom Balian loves) in order to prevent her marriage to the fiance, Guy of Lusignan, who will certainly plunge the Kingdom of Jerusalem into ruin with his thirst for war.
William Thatcher in A Knight's Tale is determined to not only be a knight when he is in fact a peasant but to defeat his jousting opponents and win London's World Championship.
Lady Hawke: Captain Navarre has this down to the time period; a heroic and pious man who uses a sword to fight evil.
Subverted for laughs in One Fine Day (1996):
Jack: You're asking me for help? Melanie: It would appear that way, wouldn't it? Jack: I'll agree if you'll say, "Jack, please be my knight in shining armor." Melanie: Jack, don't be a shithead, go rescue our kids. Jack: This is hard for you, isn't it?
When Vivian of Pretty Woman was a little girl, she would pretend she was a princess... trapped in a tower by a wicked queen. And then suddenly this knight... on a white horse with these colors flying would come charging up and draw his sword. And she would wave. And he would climb up the tower and rescue her.
Prince Charming, a 2001 made-for-television film starring Sean Maguire, is the story of a prince who gets turned into a frog because he dashingly rescues a damsel in distress who starts trying to reward him.
The titular protagonist of the RoboCop series is one modern example, though replace "shining" with "Kevlar/Titanium laminated", with nifty purple-on-blue highlights to boot. Despite his almost completely mechanical appearance, he will always uphold the law even if he has to do it by the book.
Jedi in general are a mix between (highly idealized versions of) knights and samurai. However, how much "knight" and how much "samurai" actually varies between cultures. Corellian Jedi, according to The Essential Guide to Warfare and Star Wars: The Old Republic, are actually a lot closer to European knights than the mainline Jedi, being descended directly from those who swore oaths of fealty to a Jedi Lord during the darkest days of the New Sith Wars. They're noted for being veryinflexible about the law and justice.
Jedi Knights in general are also supposed to be this (the word 'knight' is in their name after all) but as noted in the trope description, this is an ideal that not every member always lives up to. That being said, Jedi Knight Qui-Gon Jinn in The Phantom Menace will defy the council to to help supposedly "pathetic" life forms.
In Time Bandits, knights appear in Kevin's bedroom. Then, at the end, one of the sets of champions the dwarfs bring to fight Evil is a group of knights.
Tristan and Isolde has the titular British knight, Tristan, who is in love with the fair Isolde.
What's Eating Gilbert Grape: Ever since his father committed suicide and his mother Bonnie became morbidly obese out of depression, Gilbert had to become the man of the house taking care of everybody, especially his developmentally disabled little brother Arnie, whom nobody should mess with unless they want Gilbert's wrath. He isn't perfect, and sometimes chafes at everybody depending on him so heavily, but when all's said and done he loves his family. Towards the end of the movie, Bonnie gratefully tells him, "You're my knight in shimmering armor." "You mean shining," he says. "No, shimmering. You shimmer."
