Why would it be important to be prepared to give away your accounts information? Different emails providers have different policies. Google allows your next of kin to access your correspondence if they produce a proof of death ; Hotmail does the same, and asks the next of kin to show they have power of attorney. YahooMail.. simply erases your mail history.
Hence, it may be easier for everyone to get access to your mails and execute your will concerning the future of these assets.
Along with the usernames, passwords and emails potentially linked to the accounts, be also prepared to write down the security questions. Your loved ones may be or may not be the one knowing ALL of the details contained in the security questions, leading to an easy recovery of the accounts.
However, do not hesitate to segregate your different passwords in separate, password protected, lists, depending on the beneficiary of your goods. You can then store the different lists on a common storage medium (online storage, physical medium, …) and to limit the access to this resource only to your executor. It’s the same as putting different boxes with different locks for different beneficiaries, waiting in a global safe which is only accessible by you and your executor, but where your executor does not own the key to individual boxes.