There are two programs that come to mind. One is the Disclosure Document program. It's inexpensive but doesn't result in publication. Essentially, you file papers with the PTO and they hold on to them for you and authenticate date of receipt of the papers if necessary. It's the proper way to mail your idea to yourself to prove conception date. Like I said, it doesn't result in publication, and -- without following up with a real patent application -- it doesn't result in enforceable patent rights.
I think what you're really thinking of is the Statutory Invention Registration (SIR) program. What people used to do (prior to publication of applications became available) was convert their application to an SIR when essentially giving up in the PTO and trying to leave a defensive publication (a publication preventing others from getting a patent). The problem is that the fees are steep (not sure why).
The cheapest way for defensive publication through the PTO is the file a real patent application (not a provisional) and allow the case to be published at 18 months. Then, just don't respond to any correspondence from the PTO. The application will eventually become abandoned but will have been published.
A cheaper way is to simply publish it yourself on the web. Will it necessarily turn up in an examiner's search when evaluating another's application? No, it might not turn up. But it's still effective protection from any subsequent invention by another that results in a patent.
It would help to get your site into
the wayback machine for date authentication purposes (and/or keep a date-stamped, witness-signed hardcopy of your site on file). I have one of those around here for my old servicemark tag-line: "Turning caffeine into patents since way back in the 1900s." Of course, if I ever get challenged on that, I'll have to find it.... ;-) Please keep yours where you can find it.
Regards.