It's a lot like Wakabamark, with a couple extras.
underline = __underline__
strikethrough = --(strikethrough)--
italic = *italic*
bold = **bold**
bold italic = ***bold italic***
^superscript = ^(superscript)
_subscript = _(subscript)
monospace = `monospace`

• unordered
• list

and

1. ordered
2. list

For the lists, prefix lines with asterisks or dashes or something, or number them.
There are a couple others but they're more or less "easter eggs".