Linqdb is nosql database for .Net. It is based on Leveldb and adds some higher-level features. It's goals are:
be programmer/devop friendly
Read the docs
Linqdb comes as one-process embedded database as well as standalone server with client. The api for both versions is identical.
Leveldb efficiency combined with high-level features like entities, linq-style operators, search and so on.
Free for non-commercial use.