Mongodb is somehow different, because I am hearing more and more about it actually being used.
Instead of relational tables, mongodb stores json documents – the gain – object mapping is pretty straightforward, so there’s no need for an additional layer between your code and the db (like O/R Mappers in case of relational databases). I’m sure that experts on this can give a few more examples, but I’ve just touched the subject.
Anyway, for real life usage, there’s a load of drivers, so it looks like it could be a promising subject. The downside is that (according to the docs) the latest official driver for C# does not support LINQ and that’s just sad.