I’ve been looking around….
- MongoDB seems to require a commercial license. http://www.mongodb.org/display/DOCS/Licensing
- RavenDB has quite a costly scheme. http://ravendb.net/licensing
CouchDB, seems to be free for commercial use? But requires Apache, which is a bit of a pain.
Are there any other good options for .NET?
From my understanding, MongoDB is open source and free to use. There are two license types: AGPL v3.0 and a commercial license. There are a few minor restrictions with the AGPL 3.0, so some may need to remove these restrictions with a commercial license, but most probably won’t.
So in short, I believe it is free and can most likely suit your needs.