anon.penet.fi
The Finnish pseudonymous remailer that let hundreds of thousands of people email and post to Usenet without a name — until a court asked for the key.
Johan "Julf" Helsingius started the service in 1993. Mail sent through anon.penet.fi had its sender replaced by a pseudonym like an12345@anon.penet.fi, and replies to that pseudonym were forwarded back. To make that work, the server kept a table mapping each pseudonym to a real address. That table was both the whole point of the service and its weak spot.
In 1995, after internal documents were posted through the remailer, the Church of Scientology got Finnish police to demand a user's identity. Helsingius handed over a single address and shut the service down in August 1996, once it was clear Finnish law would not protect the table. The lesson stuck: later remailers such as Mixmaster keep nothing worth seizing, and the same idea underpins Tor.