BugMeNot is a database of (freely provided) account names and passwords for many sites that require registration. Instead of creating an account, you ask BugMeNot for an existing account and password that will (hopefully) work there. BugMeNot looks through its previously created accounts and returns a pairing for you to use. There is even an Firefox extension to streamline the database query and form fill.
Anonymity through shared credentials.
The FAQ warns that only 'fake' (e.g. some disposable account I might create to read some online whitepaper) accounts should be registered. Nevertheless, people do appear to be registering 'real' (i.e. those for which they receive customized service) accounts.
As an example, using the BugMeNot extension, I was able (after many failed attempts) to access the (non-paid) Last.fm account of somebody named "Zudo***" and view their profile page. I know this was a real account (and not some aggregate reflection of all previous BugMeNot users who had been given this account name and password) because I was able to view which songs they had been listening to over the last week, so they must have installed the Last.fm plug-in into their music player.
Message to Zudo*** - Devo?