I'm not sure what you mean about double-spacing either. Line breaks (<br />) are made by adding two spaces to the end of a line. If you don't like that, try GitHub-flavored Markdown [2].
I was only aware of the last one. This has the capability to be used as a replacement to html, not just a limited subset. It can also do syntax transformations like the table, which also accepts other syntax transforms and tags. Markdown can't take html attributes either.
[1]: http://daringfireball.net/projects/markdown/syntax#links [2]: http://github.github.com/github-flavored-markdown/