Best FREE Duplicate Content Checkers

In my search for the best free duplicate content checkers.  Check for plagiarism.

Dupli Checker

Caught the same one line of duplicate content as Small SEO Tools.  These two content checkers look to be using the same engine to scan and analyze text.


Limited amount of searches with free version

Nice interface.  Displays underlined text that is duplicated

Small SEO Tools

Key Search

Looks to be very accurate but does not display the source.