drop.oc — leave something for one person, once.
The first open burns it. Links die in 24 hours.
Write a note, or attach a small file (up to about 200 KB). You get a one-time link — whoever opens it first gets the drop, and it's gone.
A password, a code, anything text. Same deal: one link, one open, then it's ash. For real end-to-end encryption, use crypt.oc instead — drops are stored as-is on the server until opened.