[andrewdavey]

RavenDB http://ravendb.net/ is an excellent document database written in .NET. It leverages lots of LINQ and C# 4.0's dynamic abilities to provide a really clean API. I'm sure it'll soon be taking advantage of the new async stuff coming in C# 5.0 as well.

It's open source as well. Well worth a look.

[hello_moto]

I heard about RavenDB but doesn't seem to gain a lot of followers among .NET developers. MongoDB seems to be the choice for .NET communities.