A completely managed database solution?

There's also a C# port of SQlite (with a FAQ here) here with an intro by Miguel de Icaza

I don't have any experience working with it, but I've heard good things about db4o.

There are numerous, many dead databases, and some commercials. After some looking through I found next open cored interesting to experiment with:

https://github.com/hhblaze/DBreeze

https://github.com/mbdavid/LiteDB

https://github.com/Alachisoft/NosDB

https://github.com/AnthonyLloyd/Fsion

https://github.com/DevrexLabs/memstate

Not managed:

https://github.com/Microsoft/FASTER - dozen lines of C++ code (easy to replace with C#)

https://github.com/ravendb/ravendb - few compiled libraries for all platforms with most code C#