From the website:
A portable app is a computer program that you can carry around with you on a portable device and use on any Windows computer. When your USB flash drive, portable hard drive, iPod or other portable device is plugged in, you have access to your software and personal data just as you would on your own PC. And when you unplug the device, none of your personal data is left behind.
For anyone that frequently needs to use many computers and likes to keep their own data private, Portable Apps is for them. Being in IT support I am always on other users pcs. Very often I need a program that is not installed on that pc. There are many portable apps available from FTP, ssh , web browsers, Open Office apps, music and video players to other very useful utilities.
At any given time I have at least 2 USB flash drives on me and one always has my portable apps installed. There is a portable version of FileZilla that I use quit frequently.
There is a nice Portable apps Menu app that will show you all of your installed apps (similar to the windows start menu). It should run as soon as you plug in your usb key.
Here are two screen shots of portable apps. Notice that they look almost exactly like the non portable versions however these apps do not leave any data or files on the pc that you run it on.
File Zilla Portable
The developers of the individual apps are constantly updating the portable apps with bug fixes and new features so I usually check the site once a week or so to download any new versions of the apps I use.
There are also new apps that under development. You can searh their forums http://portableapps.com/forums to find out if one of your favorite apps can be made portable.