There's a reason we call the trope "Draco InLeatherPants". There's something about tanned cow hide that's cool and extremely sexy to many people.
Leather's Badass associations originally stem from leather being the garb of choice for bikers, who wore it for practical reasons. One of the quickest ways to show that someone is Badass or sexy in a "bad boy / girl" way is to dress them up in shiny black leather.
It will be black leather the vast majority of the time. Other colors just don't have the same effect in most cases, though white and red do sometimes work, as does the traditional brown. For increased badassery, have the black leather lit by a sodium lamp: Black leather, deep green highlights.
See also Spandex, Latex, or Leather, Pretty in Mink, Dominatrix, and Leatherman. Compare its polymer sister trope, Dressed All in Rubber.
Surprisingly enough, though, it's mostly his protagonists that wear leather. Hell Kaiser Ryo in Season 2 of Yu-Gi-Oh! GX plays it straighter than most.
Tsume in Wolf's Rain appears to wear a ripped leather jacket and pants. Of course, that's just part of his disguise (look at the title again).
Naomi Misora and Mello in Death Note. Interestingly enough, Word of God says that Mello ended up dressing like that because the artist put him in clothes that he likes drawing.
Edward Elric from Fullmetal Alchemist is never seen without his leather pants and occasionally even leather jackets. He's regularly mocked for his tacky taste and gaudy fashion sense.
Fate Testarossa of Magical Girl Lyrical Nanoha has been fighting in a leather one-piece since she was nine. Before her mother, Precia, was first shown to viewers, many of them wondered why she let her child dress like that. Turns out that, besides being the worst mom ever, she also dresses like that, and her device's most commonly used form is a whip.
Surprising amount of characters in Gunnm/Battle Angel Alita, starting from the heroine herself and Zekka, even though they technically don't really need clothing at all, most of them being Hollywood Cyborgs. They usually wear it for its sheer Badass appeal — there's the definite correlation between the amount of leather and general badassitude of the character in the series.
Fight Club: Tyler Durden wears red leather and goes as far as to say that in an ideal world a man will own just one set of clothes for his entire life, made of leather. He's supposed to be a subconcious representation of the repressed protagonist's idea of masculinity.
The alternate title of the 1968 British-French film The Girl on a Motorcycle is Naked Under Leather.
In Eragon, just about everyone of importance wears leather of some kind. This is extremely inaccurate. Justified, though not trope-wise, in that the wardrobe production was given little time to make the vast amounts of costumes needed, and someone had bought large quanties of leather; they had to use what they had, budget wouldn't allow more costs.
Well it wasn't for logic, the elves of the series were of the "die-hard vegan, would die rather than kill an innocent bunny" variety. Somewhat justified as they do make use of animals that die naturally, one elven character even wearing a headdress of swan feathers gather from dead swans and feathers swans had lost.
Caine, the Badass protagonist from Heroes Die always wears black leather.
Geralt is almost never seen out of his favorite leather jacket with lots of silver trim and studs, and matching pants. Somewhat justified in that leather indeed was a material of choice for a combat wear in The Dung Ages, and silver is useful in his line of work, but we sometimes almost get to hearDandelion cringe at his friend's fashion sense.
Kate Daniels bought her black leather jacket in her teenaged "bravo" days, when she was deliberating trying to invoke this trope. Developing into a more mature, sensible adult doesn't stop her from wearing leather at every chance, though, because leather affords some protection from attacks.
In her first appearance in Time Scout, Margo Smith is wearing a nigh-heart-stopping tight leather miniskirt. A character later describes her as "doe eyed one second, hell bent for leather the next".
Harry Dresden's distinctive, enchanted black leather duster comes up frequently in The Dresden Files, though the badassery of its leather is rarely explicitly stated. It is brought up when Harry needs clearance to enter a decidedly shifty night club, but Thomas is able to get him in because he's "A man in a black leather coat."
Angel. When Wesley makes his return as a "rogue demon hunter", he's wearing all leather. He has to change out of them though, as they chafe his...legs.
And Illyria. Oh dear Christ, Illyria.
Angelus's trademark is his leather pants, which Spike makes a hobby out of making fun of.
Parodied in one or more The Benny Hill Show sketches. Benny plays an actor playing a bad boy dressed all in leather while filming a scene. Not only is it not sexy (cause, y'know, it's Benny Hill), the leather squeaks so much whenever he moves that it drowns out recording the dialogue, especially when he sits on a leather covered chair.
Ross on Friends tries to look cool for a date by putting on tight leather pants. Then he goes to the bathroom and finds that he can't put them back on.
Derek Hale in Teen Wolf. His beta wolves seem to be picking up this habit, too.
Was described as a "long, hot slog in leather" and Claudia Black has been known to say that all she needed to perk her up when she was tired or burnt out was Ben to walk in wearing his leathers.
There is a reason Warren Ellis described Farscape as "one American's descent into Australia's S&M scene" (aside from him being, y'know, Warren Ellis).
Stargate Atlantis did this increasingly so as the series went on. By season five virtually the entire main cast are wearing as their standard off-world outfits: black leather jackets (McKay, Keller, and occasionally Sheppard), black leather jacket and pants (Teyla), or black leather trenchcoats (Ronon). Then there's the Wraith...
KELLY: Nothing says "Bridesmaids are easy" like short black leather.
The Ninth Doctor's signature black leather jacket on Doctor Who. It's a noticeably stripped-down, no-frills look compared to his predecessors, which says a lot about him from the get-go.
More precisely, it's a Kriegsmarine Jacket. Oddly enough, absolutely no-one bats an eyelid at him wearing the jacket of a U-Boat Captain in the middle of the London Blitz, aside from Jack Harkness who lampshades his bad choice of wardrobe.
Parodied in "Smith and Jones" in which a "Slab" slave is supposedly leather all the way through. The Doctor's rection: "Someone's got one hell of a fetish."
Drop the Dead Donkey. On seeing Helen's girlfriend — a lesbian motorcycle courier — Dave provides the above page quote, while Henry is ecstatic when she straddles her motorbike.
Both heroes and villains in Blake's 7. Apparently an S&M shop made the outfits, as they had the most experience in working with leather (there's a cute anecdote about the shopworkers frantically hiding the porn and the nipple clamps when Michael Keating (Vila) turned up for a fitting with his little daughter in tow).
The Mord-Sith in Legend of the Seeker (and the Sword of Truth novels) are a blatant dominatrix archetype. With Cara a main character in season two, every episode is chock full of skin-tight red leather goodness (and somehow even more cleavage than Kahlan).
Future Claire in season 3 of Heroes has quite the black leather ensemble.
Seldom has black leather and studs looked better than on Evillll Prince Dirk Blackpool. But it just doesn't work on his brother Prince Geoffrey.
When Ryu Terui first showed up in Kamen Rider Double, he always wore a leather pants/jacket outfit in either red or black. After defeating Weather, he switched to some kind of lighter material with leather patches on the shoulders and joints in red or blue.
In one episode of That '70s Show Kelso gets a leather jacket because he wants to look like Brando in The Wild One. Instead everybody laughs at him for looking like Fonzie. When he throws it away in anger Hyde puts it on and immediately becomes irresistible.
Alex Krycek of The X-Files is rarely seen without a black leather jacket. It's rarely the same one from episode to episode, though, since they tend to get ruined when someone (usually Mulder) beats him up.
Mulder himself has been seen wearing a leather jacket.
Rumplestiltskin's leather pants have their own fanbase.
Sarah dons a black leather catsuit in several episodes of Chuck. Fanboys were very happy.
Eric Northman from True Blood appears to wear a different leather jacket in every single appearance (when he isn't stark naked or wearing a tracksuit). This is especially notable as he wears the exact samesinglet and jeans every episode. Most of the vampires fall into this to some extent; justified in that a guy in a t-shirt and shorts isn't very seductive or intimidating, same with a girl in a flowery skirt.
Black leather is almost an icon for heavy metal music, in particular for the New Wave Of British Heavy Metal in the 80's. In particular, the band that popularized it was Judas Priest, and the Trope Namer is their song "Hell Bent For Leather" from their 1978 disc Killing Machine.
The Far Side had a rebellious young cow wearing a leather jacket—"just for the shock value," his parents explain.
The unnamed character in Bally's Eight Ball wears a brown leather jacket, as befitting an unauthorized copy of Happy Days' The Fonz.
Leather armor is, fittingly enough, the armor of choice of rogue classes, especially in Dungeons & Dragons. Sadly, real leather armor is boiled into rigid pieces, and doesn't look or behave anything like leather clothing.
In Vampire The Eternal Struggle, the Leather Jacket is an equipment card. It prevents all damage from a single attack, and is then destroyed. By comparison, the Flak Jacket can be used every round, but only prevents 1 damage.
Tequila Yuen sports a brown one and pulls it off. Maybe it's because he is Chow Yun Fat...
Angel Devoid, with a really Badass sounding walkloop to boot.
Psycho Mantis. The man is a villain, true, but given that his personal outfit is a few yards of leather away from being a gimp suit does help in solidifying his place as one of the most badass bosses in all the games. An interesting example in that, going by his death scene, he abhors sexuality.
A lot of the characters in Brütal Legend, but the Fire Barons, Zaulia, and Tainted Coil are standouts.
This trope especially applies to Lita Halford, who wears a sexy outfit that consists of a midriff-baring, cleavage-enhancing leather halter top and skin-tight leather pants.
Special mention to the Baron leader who is modeled after Rob Halford, the man considered as having brought the leather fashion into Heavy Metal culture.
Ezio Auditore of Assassin's Creed II back in his younger years wore leather pants that left only a bit to the imagination. It is actually Lampshaded by one of the bystander dialogues.
K' and Kula Diamond from The King of Fighters.
Flustered Lady: Such tight leather pants.
Albert Wesker of the Resident Evilused to wear just normal-looking combat gear in the first game and Code Veronica. However, he shows up in 5 wearing head-to-toe black leather complete with a leather Badass Longcoat, just in case the viewer still wasn't completely sure about his alignment.
Squall Leonhart from Final Fantasy VIII has his famous black leather and lion fur jacket and matching pants. On the opposite side, The Rival Seifer wears a white leather trenchcoat.
"Given that Morrison seems to be wearing the same pair of leather trousers in every picture there is of him, and given how difficult it is to clean leather properly, assuming you even try, I don't imagine you'd have wanted to stand downwind of the Lizard King."
Gerard Way from My Chemical Romance used to wear leather jackets until they fell apart. See above, speculation on how bad Jim Morrison's pants smelled.
"I have a leather jacket that could turn your stomach..."
Joan Jett is a good female example of this- she's frequently seen in leather pants or jackets, or head-to-toe leather outfits. (Though if the leather flag sticker on her guitar is indication, this may be more than just a fashion choice.)
Many metal artists, including Trope NamerRob Halford with a similarly titled Judas Priest song. Ironically, Halford himself prefers pleather so as not to sacrifice innocent cattle, and the song title itself was originally used in the theme to Rawhide, though these days it's more associated with Halford and Judas Priest than anything else.
Red Commissars and Chekists of the Russian Civil War era. Their uniform included a black leather peaked cap (replaced with a red headscarf for female commissars), a black leather jacket and jackboots. Particularly Ksanka in the Soviet action movie The Crown Of The Russian Empire, or The Elusives Again.
The crew of Trotsky's personal armored locomotive train wore red leather outfits — military-style jackets, pants, jackboots, and hats that looked sort of like ushankas.
Younger panzer crews in Nazi Germany, particularly in regiments which were formed in the middle to late war years, were fond of wearing U-boat crew leathers.
Both of the above had practical considerations, as both train crews and panzer crews were working with hot machinery that could probably set cloth on fire. Plus it really did look pretty badass. So... yeah.