Depending on what version of iPhoto you are using, there is an App. called iPhoto Diet, it:
-Removes duplicates
-Removes any 'originals' (iPhoto duplicates edited images)
-Removes any 'stragglers' (Photos that are in your Lib. but not an Album)
Its very good, but due to the way the new iPhoto is indexed or something, it will not work with this version.
I think its backwards compatible with anything but the iPhoto you get with iLife '06 - which sadly I now have
its brilliant but I liked iPhoto Diet