Plagiarism defines as the act of the writings of another person and passing them off as one's own. The fraudulence is closely related to forgery and piracy - practices generally in violation of copyright laws. Software plagiarism has been an important issue in software industry for intellectual property and software license protection, especially for open source projects. Thus it is important to develop robust and effective approaches to software plagiarism detection. In this paper, the authors compare six tools for detecting plagiarism: GPlag, JPlag, Marble, MOSS, plaggie and SIM.