Q:

Is there a complete Markdown reference somewhere? I mean COMPLETE

I've been looking around at various Markdown cheat sheets and references. None of them are really comprehensive. I know this because there's a text centering syntax that looks like this:

-> Centered Text <-

And none of the pages I've found mention that. Any help would be appreciated, thanks in advance.

A:

The other comments are exactly right. While there isn't one definitive source I find I have just about everything I need from these three sources:

John Gruber's original standard
GitHub Flavored Markdown
MultiMarkdown User Guide (PDF)

While there are others GitHub and MMD are about as good as you're going since they're popular enough to have good support in Markdown parsers.
You're pretty safe to use anything in the original standard and have it work across all parsers. The additional features GitHub, MultiMarkdown and other extensions use are far less standardized. Even among the different standards specific features support may very; MMD's footnotes in particular have become fairly ubiquitous among some Markdown ecosystems like the iOS text editors.
In other words, you can play it safe and stick to the original standard or mix-and-match additional features but you need to know the tools your using so you can decide which features will work for you and your systems. 
My recommendation would be add extensions to Markdown as you need them. When you add a new system go through all your Markdown tools and find out which support those features and which don't. Make a note of them and then add the additional documentation so to your bookmarks/notes/downloads/ect.
In addition to the those three above I do also love CriticMarkup, if you have to do much editing, especially collaboratively, it's awesome. I include this because it has been explicitly designed to be parsed by a human and removed before you do the final output so it doesn't mess up your tools.
I hope that helps.

A:

UPDATE 2015-03-02:
CommonMark  aims to be the one and only specification once it reaches version number 1.0

We propose a standard, unambiguous syntax specification for Markdown, along with a suite of comprehensive tests to validate Markdown implementations against this specification
...
We're a group of Markdown fans who either work at companies with industrial scale deployments of Markdown, have written Markdown parsers, have extensive experience supporting Markdown with end users – or all of the above

For compatibility purposes I'm using as defacto complete reference Markdown syntax as is described and supported by the Pandoc tool.
For me if a feature is supported by the Pandoc markdown I can use it because Pandoc can be used to round-trip it to another tool or format if needed.
http://johnmacfarlane.net/pandoc/README.html#pandocs-markdown
For simple occasional writing I'm using as a reference features supported by the https://stackedit.io online editor.
Syntax supported by StackEdit is combination of GitHub flavour and Markdown Extra and it is described here
https://github.com/benweet/stackedit/blob/master/public/res/WELCOME.md#markdown-extra

