The recent Heartbleed bug scare might have left you wanting to delete accounts on site you don't use anymore. For a quick way to find such sites you signed up on, go to your inbox and search for the term: "Confirm your email".
When something like the Heartbleed security bug hits millions of people, our standard advice is to change your passwords across all the affected services. With Heartbleed, the list was so large that it was advisable to go back to every site you know you have an account with. If many of them are unused, it kind of makes sense to just delete them in case some other security flaw in the future compromises you.
If you are lucky, you use Lastpass to manage all your accounts, and you can just delete everything from that list. But if that's not the case, chances are you have used one or two email accounts to sign up with all these websites over the years. Searching for "confirm your email" (first with the quotes, then without) in your inbox gives you a list of most of these websites, after which it's a matter of going there and deleting your account. The trick is similar to, and inspired by, searching for "unsubscribe" to purge newsletters in your email.