Here is one: No.
InDesign Tagged Text is not XML-compatible (or hardly), so you cannot any of the usual advanced XML viewers (where one could try to work around it with some XSLT styling and a nice CSS sheet) [a]. Even if it was, it's unlikely you would get "original format (with all fonts, styles etc..)", as ID supports way more formatting options than possible with such a viewer.
So if you need such a viewer, you need to write one yourself.
[a] Maybe all you need to write is a from/to-Proper XML translator, which would at least give you some more options with 3rd party XML viewers. But a full "original format" viewer? Nah.
Sorry, but what? Why would converting InDesign tagged text to another format help? You would get a PDF with all tagged text, right?
If you want to externally edit the InDesign text, either use InCopy (which is created to do just that) or export the text as RTF, not as tagged text. Be aware there are many pitfalls with this trajectory -- it has been reported to work, nevertheless.
The more conventional method is to format your text as it is written within a database that uses its own accessible tagging system and which can also display the formatting, such as Filemaker Pro, and then the database translates its internal formatting tags into InDesign tagged text for output to file by means of replacement formulas (or "calculations" in FM parlance).