Originally Posted by Stodder
...I'm particularly interested in testing a Mobi/Kindle e-book, but don't have a Kindle or iPad.
For Kindle compatible ebooks, other than PDF, you are in luck. Amazon distributes the only app that I'm aware of that is designed to preview the display on a ereader device. It is the Kindle Previewer. Its currently at v1.5 and available free from this page (scroll down to find it):
It can be set to replicate the standard 6" Kindle, the Kindle DX, the Kindle software for iPad, and the same for the iPhone. It displays images in color, if they are color, when replicating the iOS devices and in greyscale with set for the e-ink Kindle devices.
You might also download the Kindle for PC, or Kindle for Mac if you use MacOS, software to preview your book's appearance in those apps. Its good to make sure everything looks right in those apps also, but they don't replicate the device experience the way Kindle Previewer does.