PortableApps 1.1 is an Open Source utility for Microsoft Windows that lets you run applications directly from an attached USB storage device such as a USB flash drive or Apple iPod. One obvious but useful example is carrying fully configured versions of Firefox (with your bookmarks, etc.) for web browsing and Thunderbird (with all your accounts configured) for email.

Placing the USB drive into any PC running Windows autoloads the PortableApps utility and lets you run the specially prepared portable versions of your applications. It can be thought of as a kind of application virtualization or sandboxing. You could, for example, run Firefox 2.0.0.14 on your PC (which is what I do) but have the PortableApps verseion of Firefox 3.0 on your USB thumbdrive for testing it anywhere without disturbing the PCs existing browser configuration.