Deseat.me uses your Google account to interrogate the Internet, produce a list of accounts linked to the Google account and provide direct links if you wish to delete them.
It is designed by Swedish developers Wille Dahlbo and Linus Unnebäck. The site says: “Privacy and data security is something we regard as extremely important. In fact, it’s our number one focus from beginning to end. That’s why we built it to run on your computer. So basically, the only thing you're telling us is what accounts you want to delete. That’s it, and since we use Googles OAuth protocol, we don't have access to any of your login information.”
I tried the service, and it certainly found a few sites I had long forgotten. But remember it only covers the Google world – it won’t work if you use a Microsoft or other email account. Google also has a My Activity Page that will give a listing – spend some time there as it is an eye opener.