The latest entrant into this market is Timeature. This simple app allows you to edit the date that is associated with a picture in Aperture, a field that is not user editable. Why would you need this? Aperture fills in this field with data for the EXIF information including in the picture it is importing. If the picture doesn't have this information it uses the date of the import, so you can see that some pictures might be incorrectly marked.
Timeature is shareware, so if you find it useful cough up some dough.