Anonymous remailer and Usenet posting service, formerly operated by Julf Helsingius' company Penetic. It became very popular in its time.
Later, someone posted sensitive material from the Church of Scientology to alt.religion.scientology. Naturally, CoS wasn't too happy about this and managed to get a search warrant... the police found out the user's real address, but it was too late.
The service itself wasn't terminated immediately after Julf decided to take it down; it just refused to take new users. All old users could still be reached for short time.
Originally, the users were just identified as anXXXXX@anon.penet.fi; the server issued a numeric E-mail address for each of the users, so that it could forward replies. (This is why it had the Co$ memo poster's E-mail address on its hard disk - most anonymous remailers these days don't allow replying, which is kind of shame.)
After the shutdown, users could be reached as naXXXXX@anon.penet.fi.
The anon.penet.fi's tale finally ended when spammers entered the scene. You can guess the rest.
Oh, and the poster of the memo had address an144108@anon.penet.fi, which turned out to be tc@alumni.caltech.edu in "real life" - but this revealing didn't much help CoS, because the user had already fled from there.
"Well, just this morning I have been phoned by three people who said they have become victims of the Scientologists! Just try to tell them that 'oh, I'm just your average graying hacker who just happened to build that infernal machine...'"
- Julf Helsingius (at TIELI mailing list - out of my memory)