As far as the Markdown syntax is concerned, if you want to get that detailed, you'll just have to use HTML.
<a href="http://example.com/" target="_blank">Hello, world!</a>
Most Markdown engines I've seen allow plain old HTML, just for situations like this where a generic text markup system just won't cut it. (The StackOverflow engine, for example.) They then run the entire output through an HTML whitelist filter, regardless, since even a Markdown-only document can easily contain XSS attacks. As such, if you or your users want to create _blank links, then they probably still can.
If that's a feature you're going to be using often, it might make sense to create your own syntax, but it's generally not a vital feature. If I want to launch that link in a new window, I'll ctrl-click it myself, thanks.
I believe that all the previously proposed solutions (apart from those that require specific implementations) result in the comments being included in the output HTML, even if they are not displayed.
If you want a comment that is strictly for yourself (readers of the converted document should not be able to see it, even with "view source") you could (ab)use the link labels (for use with reference style links) that are available in the core Markdown specification:
http://daringfireball.net/projects/markdown/syntax#link
That is:
[comment]: <> (This is a comment, it will not be included)
[comment]: <> (in the output file unless you use it in)
[comment]: <> (a reference style link.)
Or you could go further:
[//]: <> (This is also a comment.)
To improve platform compatibility (and to save one keystroke) it is also possible to use # (which is a legitimate hyperlink target) instead of <>:
[//]: # (This may be the most platform independent comment)
For maximum portability it is important to insert a blank line before and after this type of comments, because some Markdown parsers do not work correctly when definitions brush up against regular text. The most recent research with Babelmark shows that blank lines before and after are both important. Some parsers will output the comment if there is no blank line before, and some parsers will exclude the following line if there is no blank line after.
In general, this approach should work with most Markdown parsers, since it's part of the core specification. (even if the behavior when multiple links are defined, or when a link is defined but never used, is not strictly specified).
Best Answer
Showing gifs need two things
1- Use this syntax as in these examples
Yields:
2- The image url must end with gif
3- For posterity: if the .gif link above ever goes bad, you will not see the image and instead see the alt-text and URL, like this:
4- for resizing the gif you can use this syntax as in this Github tutorial link
Yields: