Markdown.
With the amount of apps and services that support this markup language, it is, without a doubt, at a high point in terms of popularity and usage than ever in its life.
And I'm willing to bet it will continue to grow in an upward trend.
However that concerns people, such as Jeff Atwood, who feels that there should be an official specification as well as validation tests to to make sure every thing in compliance.
In other words, Jeff (and others) want to treat this like an official language — such as HTML.
I think it's a great idea, if John Gruber is willing to play along. However, I do wish elements of Fletcher Penny's MultiMarkdown fork would be considered as well (tables, footnotes, etc.).