from the BPL Online Registration and eCard FAQ :
Can I register for a library card online? yes!!
For a limited time, Massachusetts State Residents can sign up for a temporary (6 month) Boston Public Library eCard via the web!
What is an "eCard"?
BPL eCards are virtual 6-month library cards that allow users immediate access to all of the Boston Public Library's electronic resources, including magazine databases, downloadable audio, video and music. eCard users who wish to check out library materials will be asked to upgrade to a standard BPL card (see below).
Who is eligible for an "eCard"?
Anyone who lives, works, attends school, or owns property in the Commonwealth of Massachusetts and is at least 13 years of age may register for an eCard.
(thanks to my student Pen for the info)