dupeGuru PE. Plus it's open-source. :)
dupeGuru Picture Edition (PE for short) is a tool to find duplicate
pictures on your computer. dupeGuru PE is a big brother of dupeGuru.
It works like dupeGuru, but is specialized for duplicate pictures
matching. dupeGuru PE runs on Windows, Mac OS X and Linux.
dupeGuru PE is efficient. Not only can dupeGuru PE find exact matches,
but it can also find duplicates among pictures of different kind (PNG,
JPG, GIF etc..) and quality. On Mac OS X, dupeGuru PE can scan your
dupeGuru PE is customizable. You can tweak its matching engine to find
exactly the kind of duplicates you want to find. The Preference page
of the help file lists all the scanning engine settings you can
dupeGuru PE is safe. Its engine has been especially designed with
safety in mind. Its reference directory system as well as its grouping
system prevent you from deleting pictures you didn't mean to delete.
Compare any picture format. dupeGuru PE supports JPG, PNG, TIFF, GIF
and BMP formats. All these formats can be compared together. The Mac
OS X version of dupeGuru PE also supports PSD and RAW (CR2 and NEF)
Your iPhoto and Aperture libraries are supported.
Do whatever you want with your duplicates. Not only can you delete
duplicates files dupeGuru finds, but you can also move or copy them
elsewhere. There are also multiple ways to filter and sort your
results to easily weed out false duplicates (for low threshold scans).
(answer is copied from https://superuser.com/questions/311633/free-visual-similarity-image-for-a-local-hard-drive-search/453251#453251 - all credit to enter link description here)