My swiss army knife of document formats is a program called pandoc. It's absolutely fabulous, powerful, and not too complicated if you don't need to do anything too complicated. It will guess input and output formats from file extensions, or you can be more specific if you need to. Today's command looks like this:
pandoc article.md -o article.html
You won't be surprised to hear that this command takes a file called
article.md which contains my markdown, and outputs a file called
article.html containing the HTML. I also use this to convert between all the various text markups, HTML, but also PDF and office document formats - it's a very comprehensive tool!