MongoDB could be an example, but it introduces yet another empire. Redis is also easier. The key is what are you designing against. If you design against a DB

MongoDB could be an example, but it introduces yet another empire. Redis is also easier. The key is what are you designing against. If you design against a DB, then you may find that scaling beyond a single host with gotchas. But, if you have the discipline to keep everything within a document, then you can scale up easier as the relationships between documents is more relaxed. However, cross document indexing and what-not creates more problems, and that in and of itself is an interesting challenge.