I personally always have one USB stick with me that has a live usb boot of Fedoraon it, but I just saw the new video from Linus tech tips and thought about extending it a bit.
He mostly talked about windows tools, but I think I will add
I used to carry Tails around on a tiny USB drive in case I wanted to do something on an untrusted machine, but a) the USB device I chose got stupidly hot, and b) I never actually needed to use it since I carry a smartphone everywhere, so I stopped.
Definitely, besides playing around with it and booting it on random Laptops, I revived two notebooks on the go. One had a broken windows install(somehow the main C drive nuked itself into 8 separate partitions) and she was still running Linux 1 month after that so I guess a win, and the other windows “Laptop” was super laggy(3-4 Gib of ram), but I granted the machine another two solid years(one time laptop run out of power during an big update, rest was great).
Not OP, but I carry one from Star Tech. They are pricey, but if you need them, the are great.ine also has mass media support, so I can use a iso on my pc and boot the other computer to it, great for setting up servers when ilo/idrac decides to give you issues.
I didn't realize you can have an OS ISO and other programs on the same USB stick. I thought the live boot ISO had to be the only thing on the stick (or multiple ISOs using Ventoy).
With Ventoy you can have additional files on the same partition. However Ventoy scans everything on that partition, so additional files can slow it down. I recommend creating a directory, say named "Files" and put an empty file ".ventoyignore" into it which makes Ventoy ignore that directory and all sub-directories.
I got the most use out of my porteus install, mainly with gnome disk installed and testdisk for the dd failures, it being persistant and having a 32bit version for very old machines
I usually just carry around a few ISOs with Ventoy. More specifically, a Win10 ISO, Ubuntu ISO, Arch ISO, Tails (not iso but who cares), and a few others that I wanna be able to install if I ruin my current system to the point I can't boot it properly. I just noticed I still need a Fedora ISO. After watching the LTT video too, I'm looking into all of those cool utilities too, don't have any recommendations yet tho.
I personally don't have a dedicated toolbox USB anymore, since any repairs I do now are in my home and I can just set up what I need when I need, but when I did have one I used SARDU to handle setting it up to boot into a grub menu that I could use to launch multiple live boot environments.
It was real handy to have Hiren's Boot CD, a couple live boot antivirus tools, live boot gparted, a live boot lightweight linux like puppylinux, and a live boot of a more "standard" linux like Ubuntu all on the same USB. Just boot to it and select which one you need from the GRUB menu.
Besides live-boot utilities, I worked on Windows most of the time, so I'd have the sysinternals suite handy, a few ninite installers exes for quick batch installs of standard programs, and a kludged together set of portable apps.
There's a handful of "portable app" launchers for Windows, and each framework has it's own library of compatible apps. I mixed and matched from a few of them with some programs that had official portable installs. I think the frameworks I used were "PortableApps" and "Liberkey".
I had a portable hardened Firefox, a portable PDF reader (I think SumatraPDF), Notepad++ (has an official portable mode), 7-zip or Peazip (can't remember, one has official portable mode), Bleachbit with the extended application support configs, the LargeAddressAware patcher (to allow 32 bit programs to use more that 4GB of RAM), and I think Teracopy had a portable mode as well.
If I had to set one up again now I would probably see if there was a portable way to carry Powershell around as well. There's a ton of stuff on Windows that's just easier to config and troubleshoot through it. I also might include some Windows 10 and 11 debloater/privacy configuration tools too, but I'd need to do some research on which ones are actually useful/good.