This is exactly what Bugmenot is made for. You can search per url and the site will give you logins (if there are any) for that website. Of course you can also submit your dummy accounts to this site so other people can use.

Even better is the fact that for all the Firefox users there is an Bugmenot Extension which is really easy to use.
You just right click on an login form and then select “Login with Bugmenot” and the extension automatically looks for login data on the bugmenot website.
And if an account doesn’t work the extension will try out the next account until you are logged in or until there are no more accounts. This site and the extension saved me allot of hassle on different occasions, and hopefully it will also be the same for you.