• A free challenge method
  Theory and discussion
  Another version with a better printout for the instructor
  Yet Another version with printout which can make it easier to evaluate
  

• Turnitin.com a well known commercial service for higher ed (see some more immediately below)
  Plagiarism.org an educational resource developed by Turnitin.com preventing plagiarism iThenticate the commercial (non-edu) version of turnitin

  Similarity Check "powered by iThenticate is an initiative started by CrossRef to help its members actively engage in efforts to prevent scholarly and professional plagiarism." It's a project of a group of scholarly publishers.

  turnitin for admissions was a version for use by Admissions Offices used for online admissions

• "A Web-Enabled Plagiarism Detection Tool" by Colin J. Neill and Ganesh Shanmuganthan in the Sept-Oct 2004 issue of the IEEE IT Professional pp 19-23.

• An interesting different approach - Chegg's (free) Citation Generator which, by making it easier for the writer to cite sources, can help avoid accidental plagiarism. It also reminds the writer of the need to cite sources
  Two style guides from Chegg which include citing are
  
  • MLA guide
  • APA guide
  Chegg offers additional free services such as their Learn which provides helpful materials in a number of (mostly STEM) disciplines. (Chegg has been a somewhat contraversial company as its "help" services are sometimes used by students to cheat.)

• Copyleaks plagiarism checker (has other services available on that page)

• SafeAssign is a plagiarism prevention service with educational content from Blackboard^TM
• CROT provides a "FREE plagiarism detection service for your Moodle server"
• WCopyfind "an open source windows-based program that explores a collection of documents, looking for matching language.
• Plagiarism Checker X takes a document you have and checks sentences via "all leading search engines"
  
• Free Plagiarism Checker "Multilingual, professional, online"
• Top 10 Free Plagiarism Detection Tools For Teachers
• "A cloud-based tool for assessing originality" VeriCite (now a part of Turnitin)
• A free plagiarism detection tool
• Viper Plagiarism Scanner has a free version
• Unicheck was UnPlag.
• PlagScan
• ITS free plagiarism checker
• A free plagiarism checker by EDUBIRDIE
• A plagiarism checker
• BibMe Plagiarism and Grammar Checker. A good discussion of plagiarism. They also have a limited free version and a more comprehensive for-pay version of the checker.
  Note: in many situations, proper citation is considered important and often the format of the citation is considered important. BibMe has two style guides to assist in proper citation:
  • EasyBib MLA
  • EasyBib APA
• Citation Machine Emphasis on proper citation and suggestions on improving writing - with a free account. (It's been pointed out that there are so many banner ads as to make it difficult to use.) "Unintentional plagiarism" detection is included in the paid account.
• Citation generation - Currently supports the APA, MLA and Chicago citation formats. A startup planning to include other text improvement features in their ad-free service.
• TypeCite A citation generator. It's supposed to be free for student use.
• Zotero "is a free, easy-to-use tool to help you collect, organize, cite, and share research." There is a charge for storage which exceeds their 300MB free limit.
• Another area where plagiarism is of concern is in blogging. Some detection tools are mentioned, especially Copyscape.
• Since paying attention to citation can help in avoiding plagiarism, here's another helper - for Chicago style
• Chegg Writing Plagiarism Checker It's selling a for-pay service, but it has lots of helpful information about what is plagiarism and how to avoid it. Included is a 5 minute video which may help inform students.
• compilatio plagiarism detection
• Plagiarism Checker

• Do your own search - put quotes in a search engine
  e.g., Go to Google or Go to DuckDuckGo and paste this into the search box: "many facets to understanding plagiarism"
  You'll find this web page near the top of the search results. (Plus some more of my web pages on this topic.)
• While any web search engine can be used to search the web, there is a search service which emphasizes "scholarly literature, including peer-reviewed papers, theses, books, preprints, abstracts and technical reports from all broad areas of research." and can be used "to find articles from a wide variety of academic publishers, professional societies, preprint repositories and universities, ..." This is Google Scholar, and can be used as an additional source. Microsoft has some academic books searchable via its Bing search engine.

There are incentives to commit plagiarism which is why we see it occurring. This led to the development of the many tools to detect plagiarism. If plagiarism detection works to detect plagiarism, then it becomes more difficult to get away with plagiarism. (Of course!) So there is an incentive to develop methods to evade detection. Unsurprisingly this has already happened, although the methods/tools aren't advertised as having this purpose, but rather as being methods of rephrasing or paraphrasing text to innocent purposes. E.g.