Markdown rocks. If you look on google the term “markdown”, google informs about 3,580,000 results. Important sites like github and stackoverflow use to post comments. Other sites has this as a foundation: wikipedia, for instance. If we agree on the fact that wiki pages are some sort of markdown documents that are processed to get the wikipedia pages. But… what on earth is markwdown???

According to Daring Fireball, one of the first website I've checked out on the subject, it

allows you to write using an easy-to-read, easy-to-write plain text format, then convert it to structurally valid XHTML (or HTML)

In other words, you can write something similar to a normal plain text document, and with this tool you can get a valid web document (html document).

What are the advantages??

From a technical point of view, I think that the advantages are many:

• Clean format: The original documents are plain text, and perfectly readable as they are. So, the human bodies out there can read it as is, understand them and change them. However, the html results are better reading, of course.
• Easy to process: As the original documents are plain text, automatic processing of these documents (scanning, searching, changing them by a program) are fairly simple. Comparing in search of changes are easily made also with well know tools (winmerge, diff, fc to mention some of them) and to store properly in source code control systems (like github, cvs, git, subversion). A perfect example is the german constitution.
• Durability: Plain text documents are as older as the computers are. So is a sure bet that, if you write something down in a plain text document, it will be readable in the following years. This is important because: what happened to WordStar?? and to WordPerfect?? what will happen to the older documents of word?? The various propietary formats that are out there and are more or less abandoned make me think that the industry need some formats that are durable in time. This can be one of them.

And the disadvantages??

• Lack of standards: Markdown syntax is not an standard. Depending of the implementation, it can allow some features and others not, and this is a problem. If you stick to a tool, and change over time, you can find that some features don't work as expcected. For me, it's a problem.
• Lack of features: In general, many of the markdown processors I've ever seen lack some features that could be interesting:
  • Graphs: a minimal support for pie charts, bar charts and line charts would be desirable, altough I find it difficult to implement and to describe using plain text
  • Diagrams: there is no support for diagrams (although some processors have implemented some sort of support). Again, a problem because it forces you to support this documents in propietary tools and then translate it to images.
• Lots of implementations: it's a language easy to implement, and many people have made their own implementations, adding confusion to the scene.

http://daringfireball.net/projects/markdown/

Written in Perl.

https://github.com/fletcher/peg-multimarkdown - Multimarkdown

It appears to be one of the most feature rich implementations of markdown: http://fletcherpenney.net/multimarkdown/. And works on many platforms: Windows, Linux and MacOs. It allows exporting to LateX (with the proper plugins) and it's GPL.

https://github.com/sirthias/pegdown

This is a library, and it doesn't work as a command-line program.

http://code.google.com/p/markdown4j/