Few days back a friend of mine asked me to suggest the best portable applications for his USB Pen Drive. So, this requirement of my friend has driven me to list out the top 5 portable applications for USB Pen Drives / Flash Drives.
Below are few of the best portable apps or programs for pen drives (in alphabetical order):
1. 7-Zip Portable
We can find most of the files in zipped format. So, this makes you to carry an unzipping program with you. This is a cool free portable application, which allows you to Zip and Unzip the files of all formats.
This is one of the most handy portable application that you would have ever come across.
2. ClamWin Antivirus Portable
Carrying a safe and infection free pen drive is always an important task. ClamWin Antivirus Portable enables you to store, transfer and move files and folders in the safest manner.
It is a free professional antivirus to keep your PC safe anytime, anywhere.
3. Foxit Reader
What do you do if you suddenly need to read a PDF files? Foxit Reader is a free portable program which allows you to read PDF files with Javascript support.
It also includes interactive form filler, annotation tools and built-in text converter.
4. Mozilla Firefox Portable Edition
Is Mozilla Firefox your favorite web browser? I know most of the answers will be yes. Then why do not you carry the safest web browser in your pen drive. Wherever you go, it lets you surf Internet in safe and secured environment. 
You can run this portable web browser on different operating systems which include Windows, Apple Mac, Linux / Unix.
5. VLC Media Player Portable
VLC is my favorite media player. No doubt, its your too. The only sole reason of its popularity is that it is so much powerful that it can play any media file regardless is format type.
This makes VLC Media Player Portable a must portable application to carry in your flash drive.
These are few of the best free portable applications for USB pen drives. These applications will prove to be very handy and useful during most of times you use your flash drive.