I do not know if this is also outside the US, but
Edited to add - Librarything is offering free memberships as well.
Haymarket books is offering ten free ebooks for two weeks.
Verso books is offering five six free ebooks until April 2. (They added another one)
Sesame Street books have several free listings on Amazon for the kindle editions.
Children's Books (I think for online only)
Scribd is offering 30 days free.
Mim's House (kids, ebooks only)
science (I think online access only)