If you use Amazon's KindleGen program to generate Kindle eBooks on the Mac, I've got a little script for you that will enable you to drag-and-drop your source material and run KindleGen automatically, rather than having to open a Terminal window and type filenames and paths on the command line.
Copy the following code and paste it into AppleScript Editor, and then save it as an Application. Once you've done that, you'll be able to drag and drop HTML files, .opf files, etc. onto it to create your Kindle eBooks.
Note: you'll either need to put kindlegen in your Applications folder, or fix the path to it near the bottom of the code.