What programs exist to find and delete "orphan files" left after deleting an Application?
If you are willing to use Terminal.app you can use find to identify files/folders which haven't been accessed for a long time.


find ~/Library/Preferences -atime +52w will list all prefences which haven't been accessed in the last 52 weeks. 
find -d ~/Library/Preferences -atime +52w -ok rm '{}' \; will ask whether you want to delete it file by file


and if you don't want to get asked for each file, run

find -d ~/Library/Preferences -atime +52w -delete