Twitter will deactivate an account only after receiving proof of a user's death, such as a link to an online obituary.
Dropbox suggests simply opening the Dropbox folder on the deceased person's computer "if you're authorized to do so." Failing that, you'll need a court order to access their files.
That's just a sampling of providers' policies, of course. For companies that have a digital-inheritance policy, it generally isn't hard to find. The best way to dig up this information is simply to do a Web search for "servicename deceased" rather than hunting through FAQs for this information.
It's important to remember that not everything can be transferred from a deceased individual to someone else. Apple iTunes and Amazon media--particularly media that is stored in the cloud--is probably not transferrable to someone else, even after the user passes away. Of course, not everything is as final as death. As with all good things that can't be put to rest, a federal case is pending on the matter.
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