One of the databases I'm experimenting with is CouchDB, it's a very interesting non-relational database that is beginning to get some traction. I haven't done much with it yet, but needed a place to store some notes, so this is it...
How to add reader security : using Futon (the GUI admin tool), select a database and then select "security" at the top of the page. You can enter a user within the square brackets by putting their username in double-quotes.
The wiki : Wiki
The Book : Book
The Company : Although CouchDB is freely available, the creators of it have founded a a company to promote and support it.