RavenDB is an OSS (with a commercial option) document database for the .NET/Windows platform. While there are other document databases around, there really isn't anything that a .NET developer can pick up and use without a significant amount of friction. With RavenDB you can get up and running in a few minutes, and that includes grasping all the basics.
RavenDB brings a lot of benefits to the table. When building RavenDB and the supporting infrastructure, the focus was always on making sure that it did the Right Thing from the .NET developer point of view. RavenDB is built by .NET developers for .NET developers.
RavenDB is also a 2nd generation document database. What we mean by saying that is that a lot of thought has been put on making sure it it does everything right. Features like Includes, Live Projections and Multi-map, and design decisions like making it Safe-By-Default, are all in to make sure RavenDB provides a real added value, and is not just yet another NoSQL solution.