Kindlegen is the main tool that eBook developers have for translating ePub into Mobi files, but as a command-line tool, it can be unfriendly to new users.
Candelabrum is a professional GUI wrapper for Kindlegen. In the Candelabrum preferences, you configure the path to your Kindlegen executable, and then you can simply drag ePubs onto Candelabrum in your dock. Awesome!
Unfortunately, Candelabrum is no longer available at candelabrum.net, and while there is a Google+ profile, there's no activity since 2011.
In the interests of preserving this fantastic piece of software, it is available in this repo so that new eBook developers can obtain and use it.
Note: If the developer of candelabrum has any problem with Candelabrum being here, please don't hesitate to get in touch.