MarkdownPad is a free Markdown editor for Windows that that can convert Text to HTML or XHTML for you easily. Dwnload the free version here.

It may not be exactly what you are looking for, but I use this service to convert my blog entries from text to html.<BR><BR>It allows you to use a convinient shorthand, such as _italics_, *bold ...

Converting Google Docs documents to HTML is easy. You can export documents as HTML, or use Docs to Markdown add-on from the Store to get the job done.

This OPML to HTML bookmark converter makes an HTML bookmark file (with browser-friendly, clickable links) from OPML you submit to it.