Coca Cola's Secret Recipe
It's no secret, here it is! Complete with the cocaine that was removed from Coke's production in 1903:


Picture and text from This American Life
The radio broadcast recording on the above link makes a very compelling case that the picture at the top of the page is really the original formula for Coke. Of course it has changed over the years; it's not exactly the same thing now as it was way back when. In addition to removing the cocaine, they've also cut the caffeine, switched from sugar to high fructose corn syrup and use phosphoric acid now instead of citric acid. The proportions of the 7X ingredients had to have changed as well over time, as the ingredients themselves have changed. Extracting oils is a very different process now than it was over 100 years ago.
The idea that the formula is ever-so-secret is a marketing gimmick. Sure, the actual formula is a trade secret, but the massive effort to keep the world's biggest secret thing is just a bunch of theater.

Photo and caption from NY Daily News
The &quot;down to the gram&quot; specifics of the production recipe today is a trade secret, but no more so than just about every trademarked, brand name food item sold today. No one person mixes and measures the whole batch (that's just practical, it's not even necessarily a security measure), so keeping the &quot;super-secret-recipe&quot; as it stands today from being published is no more difficult for Coke than it is for KFC. Nor are there highly-paid super-slueths trying to steal the formula against whom Coke needs to take extraordinary measures.
EDIT 9/20/15 This American Life has added the following to their story:

Much of the press about our story takes at face value Coca-Cola's statements that we have not found the recipe for Coke, present or past. But when asked if the company has actually checked to see if this formula matches the original formula - which archivist Phil Mooney assured me they still have - company spokespeople always politely sidestep the question. So it seems entirely possible that no one at Coke has checked. If they'd checked to see if the formulas match, why not say so?
Phil Mooney even admitted something interesting about the recipe in our interview: &quot;Could it be a precursor? Yeah, absolutely.&quot; He then went on to express, as his opinion, not as fact: &quot;Is this the one that went to market? I don't think so.&quot;

They're just goofin.
Addendum
When I wrote this answer, I had no idea that it was going to get so &quot;big&quot; or that Skeptics.SE and Chemistry.SE had taken on the same issue. Had I known, I would have spent a little bit more time researching. I have since done quite a bit more research and I am now even more strongly of the opinion that the secret-formula of Coke is just a normal trade secret, and the hype is just hype for the sake of marketing.
Furthermore, the recipe for the 7X (that's even what Coke calls its &quot;ever so super-secret&quot; formula) in the old book pictured at the top of this answer is genuinely either the original formula or a precursor to it. At any rate, it's darn close.
If you're interested in learning more about the handwritten recipe in the book, I strongly encourage you follow this link: This American Life and listen to the recording of the radio broadcast about the book, the formula and the hype.
Early in the broadcast they make a point that I've been saying since I first got wind of this completely over-the-top marketing gimmick. One of the legends of &quot;the secret&quot; is that it is only known by two people, and they never fly on the same plane (you know, like the President of the United States and the Vice President).

“The company has always said, and as far as I know it’s true, that at any given time only two people know how to mix the 7X flavoring ingredient,” Mark Pendergrast, historian and author of For God, Country and Coke told This American Life. “Those two people never travel on the same plane in case it crashes; it’s this carefully passed-on secret ritual and the formula is kept in a bank vault.” Time Magazine

Think about that a second. If the vault contains &quot;the secret&quot; known to only the two men, and the two men died in the same plane crash, wouldn't someone at the company be able to open the vault? If the two men know something beyond that (or somethings, one version of the legend held that each man only knew half the secret) why wouldn't they put that information in the vault as well?
Furthermore, if you have a really big secret that you'll go to any lengths to keep, would you shout from every mountaintop, &quot;I know something YOU don't! Nee ner nee ner nee ner!&quot;?
So I say again, yes it's a trade secret, it's just a normal trade secret. It's not the Manhattan Project. The hype is just hype, the fact that we're talking about it now just emphasizes how effective the marketing is. Buying into the hype is just drinking the Kool-Aid.