Alex and Marty as well. Their friendship is what helps Alex conquer his predatory urges. And in the sequel, Marty has the "withdrawal" part when he finds out Alex can't tell him apart from the other zebras.
A Heterosexual Life Trio with Sid, Manny and Diego in Ice Age. (Though some would dare to say that Manny and Diego are the two main Heterosexual Life Partners because of their obvious closeness with Sid still being a part of it but the movies show that Sid is nowhere as close with Manny and Diego as the two are with each other.)
How to Train Your Dragon: Interspecies human/dragon example - Hiccup and Toothless. The whole film is built on their Forbidden Friendship after being unable to kill each other and they only get closer from there. In the television series they're inseperable, would risk anything for the other and can't stand to be apart.
Manolo Sanchez and Joaquin Mondragon in The Book of Life, they may be after the same woman but the two are best friends and constantly refer to each other as "Brother."
In fact, all of the main characters in said films come in pairs, most with the same dynamic. The most apparent being Holden and Banky in "Chasing Amy"...whose relationship gets to the point that Holden suggests a three-some just to see if they're really in love with each other.
In the movie Stick It (A gymnastics star who walked out of a national-level event for very valid and personal reasons, takes it back up and discovers it's fun. Also much snarking and funny), the main character has two goofy male friends, one of whom introduces the other as his 'hetero life partner'. And they manage to portray that, despite one of them trying on a dress when they take the gymnastics team out dress-shopping.
Captain Jack Sparrow and Mr. Gibbs are a classic example.
The Horrible Bosses movies give us another Heterosexual Life Trio with Kurt, Dale, and Nick, the three main characters. Though their (extremely) frequent bickering makes it a Vitriolic Best Buds version, the first film implies that they have been like The Three Musketeers since high school, and now, as adults, they are drinking buddies and confidants who eventually try to start a business together (with not overly successful results). By the end of the second movie, even after everything that's happened, the three of them are still best friends.
Simon Pegg's and Nick Frost's characters in both Shaun of the Dead and Hot Fuzz. And very much so in Real Life, and that friendship was simply translated onto the screen.
Shaun and Ed in SOTD is a particularly interesting case, as the movie bills itself as a "romantic comedy with zombies", presumably the romance between Shaun and his girlfriend Liz. But it's really more about the friendship between Shaun and Ed as the movie ends with them, the idea being that not even being a zombie (and in a sense death) can prevent Ed from being with his best friend.
In the Shaun of the Dead commentary, Simon Pegg mentioned that Nick had actually lived on his couch for a while in Real Life exactly like Ed does in the movie. Art imitating life indeed!
Dead Poets Society: Neil and Todd were set out to be this. Then Neil committed suicide and Todd was left heartbroken.
Marwood (credited as "...&I") and Withnail from Withnail & I. As Paul McGann said on the commentary: "It's like a marriage going wrong."
Dan Cain and Herbert West in the first two Re-Animator movies. In the first, they start out mutually suspicious but grow closer as they work together; by the second, they are sharing a house and West is actively jealous of Dan's girlfriend.
Bob Wallace and Phil Davis in White Christmas certainly counts. While at first they are only really business partners, as the movie progresses it is quite evident how much they enjoy each other's company and how thankful they are to know one another.
Han Solo and Chewbacca. They live in the same ship and travel everywhere together, for god's sake. How much more life partner can you get? They may have ended up as friends but their relationship started as a life debt after Han saved Chewbacca from slavery.
In the Star Wars prequels and the Expanded Universe, Obi-Wan and Anakin. That is part of the reason their climactic fight was so tragic. It's been commented — accurately — that they bicker like an old married couple.
R2-D2 and C-3PO, technically asexual life partners, being droids and all.
Dale, Saul and Red in Pineapple Express, making up a rare trio example. It is Lampshaded. The end of the film does not see Dale tearfully reunited with his (presumably still in hiding) girlfriend Angie, but hanging out with Saul and Red instead.
Abbott and Costello. Only on-screen. In real life, they were not particularly close (and at one point, at the height of their fame, spent a year not speaking to one another except when the cameras were rolling).
Poppy and Zoe from Happy-Go-Lucky teach at the same primary school, share a flat, are best friends, and have travelled the world together. They have have a regular very silly rapport all through the film, and it's implied that if they ever want to give up the flat, they'd buy a house together too. Throw in a few "jokey," Les Yay comments and another character thinking they actually are couple, and if Poppy hadn't started dating a social worker guy...
Billy Mack and his manager, Joe, from Love Actually. Solidified at the end where Billy returns early from a party at Elton John's house to spend Christmas with Joe, who he confesses has been his best friend for countless years, and is essentially "the fucking love of my life."
Joe: Well, this is a surprise. Ten minutes at Elton John's and you're as gay as a maypole.
By the fourth Lethal Weapon, Riggs and Murtaugh certainly qualify.
Gene Wilder and Richard Pryor in all the four films they did together: Silver Streak, Stir Crazy,See No Evil, Hear No Evil and Another You
The film Becket depicts King Henry II of England (Peter O'Toole) and Thomas Becket (Richard Burton) as heterosexual life partners until Henry made the mistake of making Becket the Archbishop of Canterbury. Becket took the job very seriously and defied Henry's rulings in favor of the church. Henry then acts like a Woman Scorned, exiling Becket and later has him killed. Becket: "He never forgave me for loving God more than him."
Viggo Mortensen and Ed Harris's characters in Appaloosa, whose relationship gets thrown for a loop with the arrival of Renee Zellweger.
Bruce Willis and Tracy Morgan's characters in the movie Cop Out, in a typical buddy cop manner.
Corey Mahoney and Terrence Christian in the SyFy channel original Soulkeeper
Roger and J.P. in the old Disney movie Angels in the Outfield. Despite only having been in a foster home with each other for a few months, and despite Roger being approximately four or five years older than J.P., they were best friends and went everywhere together. At the end, Roger refuses to allow himself to be adopted (he wanted a family more than anything in the world) unless J.P. could come with him.
Connor and Murphy McManus from The Boondock Saints. They're twins and so have spent their entire lives together; they work in the same meatpacking plant, live together, and would do anything for each other. This includes pulling a toilet out of the floor and dropping it on the head of a Russian mafia man from five stories up.
Gary and Julius from Remember the Titans become this over the course of the movie. After Gary's accident, Julius talks to Gary in the hospital about how race won't always come between them and essentially growing old together.
In Strange Brew, Bob and Doug McKenzie are temporarily split up. They immediately go through withdrawal symptoms and become completely useless until they are reunited.
Romy and Michele from Romy and Michele's High School Reunion fit this trope. Michele doesn't want to dance with the guy who had a crush on her in high school and who still loves her unless Romy can join them. Aww.
During The Avengers, Bruce Banner and Tony Stark hit it off when they're both called to help with the Tesseract problem. By the end of the movie, Bruce is seen hopping into Tony's convertible to gleefully tear off into the geeky sunset.
Tony's voice overs throughout Iron Man 3 are revealed in The Stinger to be part of a pseudo therapy session with Bruce, who points out that he's not that kind of doctor.
Tony's been this with Rhodey even longer than he has with Bruce.
Also in the Marvel Cinematic Universe, Steve Rogers and Bucky Barnes have enormous affection for each other. The strength of this relationship is highlighted in The Winter Soldierafter Steve finds out that Bucky is the eponymous villain.
Also in Captain America: The Winter Soldier, Steve bonds almost immediately with VA counselor Sam Wilson (Falcon), who in turn chooses to forgo hunting HYDRA with Fury to accompany Steve in his search for the still-missing Bucky Barnes.
Back to the Future: Marty and Doc, "Partners in Time". In the first film, Doc sacrifices himself to save Marty from the Libyans, and Marty returns the favor by ensuring that he gets the news of his impending death. Throughout the entire trilogy, they're by each others' sides, protecting each other and providing the fandom with many crowning moments of heartwarming.
James Franco and Seth Rogen in This Is the End are a subversion. Seth is good friends with James, who is more than a little obsessed with Seth, to the point of willingly sacrificing himself to a group of cannibals so that he can save Seth. Seth, on the other hand, finds the affection a bit weird. Made more amusing by the fact that Rogen and Franco actually are Heterosexual Life-Partners in real life.
Victor Creed with his half-brother Logan for their childhood and most of their adult lives, up until Logan leaves Team X in X-Men Origins: Wolverine.
Considering that Charles Xavier's and Erik Lehnsherr's friendship only lasted a few months in X-Men: First Class, it was unusually intimate on an emotional level.
Mariko and Yukio in The Wolverine. They are not related, as Yukio was a poor girl found in the street by the Yashida family, but they were raised and treat each other as if they were sisters.
Played straight in X-Men: Days of Future Past with the elderly Magneto and Professor X (the moment where they're holding hands is the closest that we've seen them since First Class), but averted with their younger selves. In 1973, Charles never once calls Erik "friend" (although Erik uses the endearment twice), which goes to show how broken their relationship is.
Moreover, Hank and Charles are each other's Only Friend for a decade, so it's inevitable that they would develop a very close bond. There are moments of non-verbal (and non-telepathic) communication between the two men, like when Xavier gently taps Beast on the chest after he mutters to Logan, "I think I'd like to wake up now." McCoy immediately understands that the gesture means, "You can calm down now, our visitor is not a threat," and he reverts back to his human form.
My Fair Lady has Professor Higgins and Colonel Pickering. Both are middle aged bachelors and seem to prefer the relationship they have with each other over one with a member of the opposite gender although this is much more evident with Higgins than Pickering.