There is a free (but not open source) edition and it's been available for some time. SQL Server Express. It has very severe limitations though, which make it difficult to recommend.
Depends on the use case. A local POS vendor is running most of their local databases in stores (1..10 registers) on the Express version. Not sure how often they archive data out of the main database, though.
There is SQL Server Express though it has a 10GB limit