The rules are defined by location. If you want all image files saved on the desktop in IMG format to be moved to My Pictures folder, all you have to do is indicate the relative rules for the program. You can create more than one at once.
Hazel doesn't just move files from one location to another, but can also do other things, such as: organize files into subfolders, add commentary or keywords, import songs to iTunes and photos to iPhoto, etc.
Additionally, Hazel includes an option for effectively administrating the recycle bin, as well as deleting program and files connected to them.
For Mac OS X 10.4 or later, including Leopard.
14-day trial version.