Linqdb is efficient and convenient database for .Net
Available as embedded db and standalone server for Windows and as standalone server for Linux.
Linqdb comes as one-process embedded database as well as standalone server with client. The api for both versions is identical.
Linqdb is based on Leveldb which results in great performance characteristics. Linqdb itself is optimized not to stand in the way.
Free for non-commercial use. Reasonable pricing for commercial use.