Jaws, who is a bad guy in “The Spy Who Loved Me” and most of “Moonraker,” but who, at the very end of the latter, helps Bond escape from the space station.
The Crossing Guard- The protagonist and antagonist switch in the movie, I’ve never seen another movie do that.
Spike on BtVS. Starts out as a Little Bad, morphs into a Big Bad, then turns into an ally to save the world (several times). Granted, the first time was to save all “the little Happy Meals on legs.” But still…
Mirage from The Incredibles. I’m not entirely sure if she quite counts because she genuinely was a villain at first (she later changes her mind about Syndrome), but she helps out the main characters in the end.
Marley (the old man with the snow shovel) in Home Alone. Kevin is scared of him at first but Marley comes to the rescue by bonking Marv in the face with the shovel, helping to foil the nefarious Wet Bandits.
To be clear, are we talking about antagonists who switch sides, or folks who were actually on the good side all along?
For that matter we can throw in Han Solo, who pretty clearly was only in it for the money and walked away from the attack on the Death Star, only to literally have Luke’s back at the critical moment.
In *The Rocketeer *Eddie Valentine, the gangster working for Neville St. Clair, changes sides and fights side-by-side with the FBI when he discovers St. Clair is actually an Evil Nazi.
To quote Anna Russell: “Love certainly took the ginger out of him.” 
In the recent British TV series of Robin Hood, Sir Guy of Gisbourne spent most of the three seasons trying to kill Robin, only to end up fighting alongside him in the uneasiest of uneasy alliances.
In Babylon 5, G’Kar starts off as a minor villain. Not necessarily “evil,” per se, but unsavory and somewhat amoral, interested only in advancing his people and putting his thumb into the eye of the Centauri. By the third season, he’s one of the show’s moral centers, an invaluable ally to Sheridan and the rest of the Alliance, and regarded as a living saint by his own people.
Londo Mollari, similarly, starts off as a buffoon, becomes the willing (if uneasy) agent of an expansionistic, genocidal empire allied with cosmic horrors, eventually seizing control of the empire himself and aligning it with the heroes in their war against the Shadows.
And, of course, the relationship between the two goes from enemies, to mortal enemies, to reluctant allies, to fast friends.
If we are talking of the former, an obvious one would be Darth Vader turning on the Emperor in Return of the Jedi.
Terminator 2 toyed with this when they first introduced Ahnold. It was rather well crafted - Ahnold approaches from one direction, the T-1000 from the other…too bad it was completely ruined by the trailers.
There’s also tons of examples where the good guy (and the audience) thinks the other character is the bad guy, but he isn’t. Bond movies do this quite a bit. cf. For Your Eyes Only - Colombo, You Only Live Twice - Tiger Tanaka, Diamonds Are Forever - Willard Whyte, etc.
Exactly. I mean, Londo is basically the primary visible antagonist of the show for a good chunk of it. By the end, I think everyone just pities what a sad pathetic individual he is and how poorly his “life” turned out. I mean, Vir is ten times the man Londo is.
I love Babylon 5.
On Farscape, Crais nearly accomplishes this. He is never fully supportive to Crichton, but he does support Talyn and even gives himself up, taking the unstable Talyn with him. This is certainly on the side of good.
Scorpius? Well, he definitely works with the heroes for most of season 4 at least. Would anyone consider him a good guy, though? He fears Scaaran control, at the very least.
I think Saffron would have ended up this way on Firefly had it run longer.
Let’s not even get into the Cylons of the recent Babylon 5. Heck, half of the human models and all of the Centurions(those with minds, anyway) end up being good and fight against Cavil and a couple other models.
Even Boomer, who tries to kill Bill, ends up being kind of good in the end.
Gaius is basically a good guy, though he is a minor antagonist in the beginning.
The only real antagonists on the show are Cavil, the Black Doctor Cylons, and the Cylons with the receding hairline. Are my wife and the only ones that call these folks these names? 
I dunno, but you probably are the only ones who call that show Babylon 5. 
The black doctor is Four (sometimes called “Simon”), but I’m not sure whether you’re referring to Two or Five as “the one with the receding hairline”. Two is the model (sometimes going by the name “Leoben”) that’s obsessed with Starbuck, and Five is the one (Doral) who looks like a boring, smarmy bureaucrat who doesn’t do much of anything (but is still always on Cavil’s side).
No, they’re totally not. When we were watching Battlestar Galactica we kept accidentally calling it Babylon 5 and now we’re watching Babylon 5 and it’s totally vice versa. Sometimes we really screw up and call either show Babylon Galactica. Seriously.
Apollo Creed in the Rocky movies.
All great examples – just for reference, what I was thinking of in the OP would be a primary antagonist in a single work (movie, book, whatever) who either evolved or was revealed to be a supportive character. I’d think it’s pretty common for the hero to woo away a secondary henchman to his side (Mirage in the Incredibles, Mini-Me in Austin Powers), and likewise for the antagonist of the last movie to team up with the hero to face a new menace to them all (Apollo Creed, Ahnold). But for the primary antagonist to change or be revealed to be something different over the course of story seems to me to be more of a writing challenge.
Reminds me of an exchange in (I think) Valor’s Choice by Tanya Huff:
“It’s like that old show, Babylon Nine.”
“No, it was called Deep Space Babylon.”
Hey, leave me alone!
:eek:
:smack:
I was talking about receing hairline(Doral), not Leoben.
Leoben is an antagonist who is revealed to be at least somewhat on the side of good. I mean, Leoben himself does stay to help start a new Earth.