If you are viewing photos in iPhoto in fullscreen mode, you can compare two photos side by side by hitting the Compare button in the lower left of the screen. What I didn't know (and didn't find any documentation for) is that when in compare mode, if you press the Command key and click on another photo in the browser at the top of the window, the clicked photo will be added to the compared photos. By repeating this process, you can compare up to eight photos.
[robg adds: This also works in the "edit in main window" and "edit in separate window" options in iPhoto. It is documented in a sentence in the help under "Compare Photos," but I still felt it worth sharing. Note that you can also replace any compared image by first clicking that image, then click its replacement in the photo browser (this is also documented).]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060910092022816