Which iPhoto folders are least useful to save space?
Before any of these, I would check your iPhoto's trashcan. Most people don't realize that the iPhoto trashcan is completely separate from the systemwide trashcan. Depending on how many photos you take, this can sometimes amounts to GB of space quite easily. I would otherwise leave those individual folders alone, as iPhoto uses those to generate the pictures that are in your iPhoto library. Depending on which folder you target (such as originals), you may end up with a bunch of question marks in your iPhoto library where images used to be. The best way to get rid of pictures is to manage them from within the iPhoto application itself.