EPUB and MOBI are one of the most popular eBook formats. Now you can crate documentation in both of this formats using Helpinator.
Helpinator created EPUBs internally, but to compile MOBI you will need KindleGen utility from Amazon:
Download and install it then update Helpinator options (Main Menu->Tools->Options, “Compilers” tab).
Now you can compile MOBI format.
Note that you will need EPUB and MOBI format viewers to preview compiled documentation. You can use Kindle Previewer for both: