When PowerPage is undergoing maintenance, the "sorry" page directs folks over to Jason O'Grady's JDOGblog.
As it happens, I just got a bit of really useful information about marketing opt-outs.
OptOutPrescreen.com is a registration service put up by the credit evaluation bureaus to mark your credit rating entries and remove them from the lists that banks use for pre-approved credit card offers. Pretty slick. Doesn't prevent organizations that you ask for credit from checking your scores, just doesn't put your name on the sorted lists that are used to get junk mail.
The Direct Marketing Association also has several Do Not Contact options. This kind of blows; if you want to register online it costs $5, but it's possible to register by mail for the price of a stamp.