Hazel is an awesome program that does a stack of things really well.
However do note that version 7 adds a Folder Changed trigger that can tell you when a file is added or removed from a folder, so for simple cases Keyboard Maestro may be able to sort you out.
Hazel has way more power in this area though, and similarly to TextExpander, if you have heavy needs they are definitely great companions to Keyboard Maestro.