Back in the dinosaur days of computing, I could, and did, carry a bootable copy of an operating system plus my key professional productivity tools -- a text editor and a telecom program, plus a few other tools, with me -- all on a 1.44MB floppy disc, with room to spare for a bunch of documents. Even, over time, tossing in a TCP/IP stack, our old friend Trumpet WinSock, if I recall correctly. In a trade show press room or at a friend's system, I could simply plunk my floppy in, reboot, and get to work.
Today, of course, no new machine has a built-in floppy drive, and between mobile devices, personal cloud storage, and easy quick access to the Web, there's far less need to tote around any software. Just stash a copy in the cloud, or grab a fresh one from SourceForge.net or Downloads.CNET.com or some other trusted site.
But "less need" isn't the same as "no need."
Depending who you are and what you do, there are still plenty of reasons -- "use cases," if you will -- to carry around apps and even entire bootable/installable OSs. For example:
No (or slow) Internet connectivity. For example, you might want to try a particular program on a computer that you're thinking of buying (at a computer store, yard sale, etc.). Or you might be on a metered-megabytes connection, like a MiFi mobile hot spot or hot-spotting from your smartphone.
You don't have permission to download software on the machine you're at.
The software requires a (paid) license (which you already have).
You'll save time by not having to search for and download a piece of software you want to use.
Privacy, safety (and paranoia). It's always a good idea to use software copies and bootable environments you trust -- particularly if you suspect/know that the target machine has malware lurking inside.
Your target system is slow enough that download/install would take too long.
The apps listed here range from a few hundred megabytes to gigabyte-ish. If you're not doing OSs, a one or two GB flash drive should do it. I've focused on Windows and Linux, but there are also portable apps for MacOS -- see, for example, OS X Portable Applications.
Most of the items listed here are "portable" apps, meaning that you can install and run them from a USB drive, rather than needing to copy/install them over to the target computer -- and will keep any configuration or temp files on the USB drive. The rest can be carried on USB drives for installation on other computers (or, perhaps, as a virtual-machine image on the flash drive).
Without further ado, here are 12 apps (and categories of apps) that are worth keeping with you at all times.
TrueCrypt (or some other encryption tool)
Cost: Free (donation suggested)
Even if you're just carrying documents with you, you may not want them publicly accessible. Some "secure" USB flash and hard drives include password and encryption protection, and some versions of Windows offer BitLocker To Go (and Microsoft offers the free BitLocker To Go reader to access FAT drives from XP or Vista... but that may be a too-blunt stick for your security druthers.
TrueCrypt is a free open-source encryption program. Encryption/decryption is "on-the-fly," meaning "data is automatically encrypted right before it is saved and decrypted right after it is loaded, without any user intervention" (according to the TrueCrypt site). TrueCrypt never saves decrypted data to disk.
TrueCrypt can work in a variety of ways, including creating a virtual encrypted disk within a file, and mounting it as a real disk and encrypting an entire partition or storage device -- like a flash drive, SD card, or hard drive.
Note, the TrueCrypt documentation advises that before removing an external drive, etc. when running TrueCrypt, you either dis-mount the TrueCrypt'ed volume from within the program, or turn the machine off, to avoid damaging the protected file(s).
PuTTY (or other SSH/terminal/Telnet app)
System and network admins may need to talk to serial devices at the command-line level. And some fossilized users like me still prefer to access email terminal-style. That requires Telnet and/or SSH (Secure Shell, basically Telnet with some security around it).
I'm fond of PuTTY because it's free and small. It's also available as a "portable app". (The main version will run directly from a USB stick, but might not retain any configuration changes.)
It's not perfect -- it's not a "True Windows App," so cutting and pasting is left/right mouse clicking, clicking on a URL takes you to the Lynx ASCII web browser in the shell; to open in a graphic browser like Firefox, you have to copy the URL, and paste it to the browser. Other SSH apps do this better... but may not be free.
TextPad (or other text editors and word processors)
Cost: About $25 for one user
TextPad is a text editor, suitable for writing articles (like this), and also as web page editor and an IDE. It handles huge files, switches among a handful of open files, and more. I've been using it as my primary writing application for well over a decade, resorting to Word only when absolutely necessary. The only thing I want that TextPad doesn't do is let me "split" views of a file into more than two windows. (PCWrite did that, one reason I still miss it.)
Portable app bundles
A "portable app" is a program you can install (or even simply copy) onto media or external storage -- e.g., a flash drive or SD card, run on any (Windows) computer, without leaving data behind (except if you explicitly saved to the computer, that is).
The PortableApps site is a compendium of 300+ free, portable apps, categorized under Accessibility, Development, Education, Games, Graphics and Pictures, Internet, Music and Video, Office, Security, and Utilities.
Many are apps -- or portable versions of apps -- that have their own main sites, e.g., TrueCrypt and Putty, which I've already talked about.
Two benefits of this site are aggregating the find-and-installs -- you don't have to click to each apps site -- and the launcher/manager.