There is, in fact, a fourth category, known to the Irish (but apparently unknown to Groom) as 'anti-plagiarism'.

Click here to access the workshop Additional resources A large number of web-sites on plagiarism and anti-plagiarism can be found through search engines.

-- Plagiarism and anti-plagiarism [online].

TOPtenreviews ranked PlagScan as best plagiarism checker in 2012.

There is even a plagiarism checker called Viper which claims to scan 10 billion resources to check whether the bespoke essay you pay for has itself been plagiarised.