I would like to see onleihe as a new source of ebooks in calibre (
www.onleihe.net).
Onleihe is a free lending service for ebooks in germany that works together with public libraries. You must be a registred user of a local library in germany, and with that account you can download ebooks out of a pool from that library via onleihe.
The ebooks are DRM-protected to make them readable for a lending period of 7 to 14 days. After that period expires, the book becomes available for the next lender.
While I am here I would like to add a feature request:
As the list of book sources grows due to the suggestions from this thread, it might be a good idea to add filters for this list. Or to have only book sources of your own locale activated by default. It is better for a new user to start with a small list and add some favourite sources later than be burried under a pile of sources from so many strange countries.