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VirusTotal.com is a website that provides free file checking. It uses up to 39 different antivirus products. Files can be sent either thru the website or via email. The variety of products used by the website allows a user to check for viruses that his own antivirus solution may have missed, or to verify against false positives. The drawback to using VirusTotal is that it can only scan submitted files, and cannot perform system-wide scans on the user's computer.
VirusTotal was selected by PC World as one of the best 100 products of 2007.