Although the ESENT API has been available in the Windows SDK for several years, I don't think enough people are aware of it. I have a blog post on the Windows SDK blog that gives an overview of ESENT and has a tiny 'hello world' program.

http://blogs.msdn.com/windowssdk/archive/2008/10/23/esent-extensible-storage-engine-api-in-the-windows-sdk.aspx

Hopefully more people will start using ESENT, instead of rolling their own data storage solution (recovery and concurrency are really hard to get right).