When the Kindle app was released some time ago, I've downloaded and after not seeing much usefulness with it because of the format/extension, I've put it on an old hard drive. Then I needed to check some effects but I could not find it again on the whole internet, hard to believe. Now that I found it, I looked whats so special about it, and will recommend the features to the Skim pdf reader developers. I liked the Sepia tone feature, along with the white on black background switch (but that can be done on OSX anyway with Ctrl+Alt+Cmd+8). Other than that I like that a page can be switched to be displayed as two columns per page. There is also text effect like editing the distance between characters, lines and also magnification.