A portable program is one that was specifically designed to not need an installation, and thus work from a flash drive. A well-designed portable program will not alter the Registry or otherwise leave any trace of itself on a computer.

There are a huge number of portable programs available, many of them free. There are portable browsers, graphic editors, media players, and programming tools and utilities.

PortableApps.com, which creates and gives away portable versions of existing free, open source software. There you’ll find your Office solution, a portable version of OpenOffice, the popular, free alternative to Microsoft Office. It doesn’t do everything Microsoft Office can do, but it does enough for most people.
Wikipedia’s  list of portable software

Some Free but good portable USB Applications

PortableApps Suite
The PortableApps Suite centralizes all of your portable USB applications in one location—a tool similar to the windows start menu. You can customize the menu with the applications you want. In addition, it conveniently organizes your files and has various other practical functions.

EssentialPIM Portable Edition
Essential PIM is an easy-to-use personal organizer that is completely stored on your USB. The application includes a calendar, to-do list and a contact manage.

GreatNews
GreatNews is an RSS reader that can operate on your desktop as well as on your USB flash drive. You can take your favorite RSS feeds with you whereever you go.

Convert
Convert is a simple unit conversion program that includes conversions for distance, temperature, volume, time, speed, mass, power, density, pressure, energy and more. You can also add your custom conversions.

Firefox, Portable Edition
You can enjoy all of the convenient features of Firefox and your bookmarks and browser extensions on any computer. Mozilla Firefox, Portable Edition resides on your USB flash drive and it will leave no personal information behind on the computer you use.

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