Online-Service: https://dillinger.io/
Have a look at to-markdown written by Dom Christie. Thanks to Galen Gidman for pointing this out!
Tip 1: If your HTML source makes heavy use of tables it might help to keep the option GitHub Flavored Markdown disabled.

This way the table tags will be left untouched. As HTML tags may be used within Markdown files the resulting syntax is still valid. It is a compromise of course because the output is not clean Markdown. For most migrations it is probably the way to go due to the Markdown table functionality beeing much more restricted compared to HTML. You would certainly end up fixing merged cells, line breaks and so on en masse.
Tip 2: When keeping HTML table tags within your Markdown file make sure two tables are seperated by an empty line. Some interpreters will mess up the output otherwise.
<table>...</table>
<table>...</table>
<br /> that is) will be replaced by two blanks ( ), which is the equivalent in Markdown and always works as expected.