Plagex
Plagex is short for "Plagiarism Exterminator". You simply paste some text, and it will search Google for key phrases and present you with the results. It's probably not as good as most commercial plagiarism detection services, but it's simple, free, and should catch the most egregious of offences.
Important notes
This software is very much in beta! Please report any bugs. Be sure you manually investigate hits before concluding that plagiarism occurred, as false positives are possible due to the work quoting a stated source, using a common phrase, or even due to hiccups in the way google searches work.
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