Note: DownloadCheck is no longer under development, and it is no longer available for sale.DownloadCheck is an application that checks for Trojan horse applications in your Downloads folder. It was written in response to the MP3Concept Trojan horse release of April 2004. See John Gruber's excellent commentary on this Trojan and the controversy/hype that it generated.
It's likely that you'll never find a MP3 or other document type masquerading as an application. Most transport mechanisms on the Internet will not preserve the resource fork of the file being downloaded. Without the resource fork, the MP3Concept Trojan, and others like it, will not work. DownloadCheck is released as a deterrent for anyone who might consider writing an exploit of this type. We hope that we'll never need to use it.