Here is an example of using this method to login to www.cricinfo.com without registration.
While login names and passwords for some of the famous websites are available, these sites continue to impress with availability of names for not so famous sites as well. However, there would still be cases where the password doesn't work anymore as somebody who copied the username/password from here just changed it to use it for himself. It saves overall a lot of time and effort required for signup, nonetheless. Typical sites where I did see it work were some of the sports sites, like the one listed above. Some online newspapers, where content was hidden by free registration. Even passwords for some forums are available. Sometimes, forums are publicly readable but some of the downloadable attachments are not really downloadable for general public. Here, our little trick can be quite useful. It won't work 100% of the time but it will on an average save a lot of time and hassles of signup.