By definition, a portable application is a program run from a removable storage device that leaves no personally identifiable information behind in the host operating system. In other words, someone crawling through a system that you recently used a portable app on, should not be able to find any traces of data in the file system or registry any more damning than the simple fact that someone ran an application. That's it. No cookies, cache files, preferences or saved data should ever touch the machine's hard drive, and if they do, they must be securely overwritten prior to the ejection of the removable media. As of 2011, only PortableApps and Ceedo are actively developing USB apps platforms.