If I'm on windows I like portable applications. I always have:

https://portableapps.com/

"Installed" and if I'm looking for a program I tend to go here:

https://www.portablefreeware.com/

first. There's also sysmenu:

https://www.ugmfree.it/

Which has way more programs, and let's not forget nirsoft:

https://www.nirsoft.net/

Someone's already mention sysinternals.

Much as I like Linux over windows, portable app have no equal on Linux.

Simple Text Editor is a very good notepad equivalent for Android. Just make sure to save, as, like notepad, if it gets closed in the background or something, you may lose data.

Thre used to be this site called tinyapps.com. It may still exist.

Edit: It exists, and is:

https://tinyapps.org/

If you're on a Mac, https://setapp.com/apps is a bundle of nice indie apps under one subscription. And even if you don't want a subscription, most of those apps are available for purchase from the devs directly, but this site is still a useful curated list of relatively high-quality small desktop apps.

The V File Viewer is excellent, and I've used it daily for many years. It's based on a DOS program called LIST.COM in the 80s/90s.

https://fileviewer.com/

When I had Windows on my work computer, FastStone Image Viewer was great for browsing photos/images.

lazpaint, musikcube, cudatext, betterbird, goldendict, sharik, windirstat, wise disk cleaner

these programmes can allow you to save a lot of CPU power and memory bytes and make your computer reliable again.

spyro memory hacker pro

cheat engine

sysinternals suite especially procexp (process explorer)

xyplorer (for viewing tree directories)