Joplin. End-to-end-encrypted, markdown-formatted note-taking app that you can either sync using the cloud, Syncthing, or use entirely offline.

Kiwix since it allows accessing some services offline (Wikipedia, iFixit, StackOverflow) where Internet cannot be taken for granted. You can host a small Kiwix server than can be accessed as a local hotspot and browser whatever packages were downloaded into it.