From MakeUseOf.com:

“This stuff can be installed on any portable drive, i.e. USB thumbdrive, PDA or an iPod. You can use them at work, school, or any other place where you can plugin your device. Check them out, you can either scroll-down for relevant category, i.e office software, email tools, messengers, games, etc. or get a ‘all-in-one’ package (all essential tools) . While there are more apps for windows users, I tried to include a mac version for each essential tool. All free. Enjoy!

What is a portable program?: A portable program is a piece of software that you can carry around with you on a portable device and use on any other computer. It can be your email program, your browser, system recovery tools or even an operating system. The coolest part about it, is that all of your data and settings are always stored on a thumbdrive so when you unplug the device, none of your personal data is left behind.”

The article lists options for 13 categories:

1. Portable Office Software
2. Portable Email/Calendar/ToDo Programs
3. Portable Text and Voice Messengers
4. Portable Media Tools (Audio / Video)
5. Portable Graphics Viewers and Image Editors
6. Portable Browsers and Web Tools (FTP, Feedreaders, Torrent clients…)
7. Portable Security Software
8. Portable Games
9. Other Portable Utilities
10. Portable Developer Tools (Techies Only)
11. All-In-One Packages
12. Portable Operating Systems (’HowTo’ Articles)
13. USB ThumDrive Tips