Hide something in plain sight
ste.gg — put a file inside a photo, a PDF or an MP3. The picture still looks like a picture.
Unicode zero-width characters — hide a full sentence inside invisible characters of a normal message. Paste it anywhere, it looks like nothing. 
Send something without a server
termbin — a paste bin that speaks netcat: echo "hello" | nc termbin.com 9999. No browser, no account, no JavaScript, and it answers with a URL. Mine returned https://termbin.com/q6ks.
croc — read a code phrase out loud, and the same file lands on the other machine, end to end. 
OnionShare — your laptop becomes a file server, a website or a chat, reachable only through an untraceable address that dies when you close the window. 
file.pizza — browser to browser, no upload step. PairDrop does the same for photos across phones and laptops on one network.
Yopass — a secret in a link that self-destructs after one read.
Read the world from a text prompt
wttr.in — curl wttr.in/Berlin gives the forecast as ASCII art, moon included. Tested: London: ☀️ +12°C.
rate.sx — curl rate.sx/btc charts live prices in text, sparklines and all. 
ipinfo.io and icanhazip — who owns an IP, and what your own IP is, both as plain JSON. 
open-meteo — the real weather API underneath the ASCII art: temperature, wind, timezone, keyless and free, ready to pipe into a script.
Learn something without leaving the shell
cheat.sh — curl cheat.sh/rsync returns the cheat sheet with working examples. tldr.sh rewrites man pages into five examples instead of forty flags. explainshell.com takes a command copied from a blog and explains every flag in plain words before you run it. 
The cursed ones, which are also the fun ones
qrenco.de — curl qrenco.de/onehack draws a QR code in ASCII, scannable off a screen with no graphics. dog.ceo returns a random dog as JSON. jokeapi returns a joke with proper keys. 
Why each of these is genuinely useful, beyond the novelty
Single-purpose, free, no account, and the output is text you can pipe somewhere else. open-meteo lets a script ask for the weather without a dashboard. cheat.sh and tldr are the two things that turn a frightened beginner into someone who opens a terminal on purpose. And OnionShare is the one to keep: a file sent through it never touches a third-party server, which matters the day you are sending something you would rather not hand to a cloud.
Why this list is worth keeping
None of these needs an install, a subscription or a login, and each one solves a problem you did not know had a single-line answer. A browser is a window. These are doors. 
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