I have replaced my ancient wordpress blog with a lean custom php framework. The blog entries, tags, categories and comments are now stored in mongodb rather than mysql. This has speed advantages in the case of a high read / low write blog as all the data for a page load is stored as a single database "document".
This is in contrast to the relational way of storing the data that needs at least 3 JOINS to retrieve the same information. Page load times dropped from ~2 seconds to 40 milliseconds (mostly due to the lean php).
mongodb