There are storage techniques such as JavaSpaces (or a commercial implementation such as Gigaspaces) which provide highly scalable, fast & secure access to objects.
There are also distributed cacheing systems such as memcached, which offer a similar approach.
Of course, neither of these are true databases, but they are things that can work in conjunction with databases to offer a large amount of horizontal scalability, given a suitable architecture. The real problem is that if you want all of the ACID goodness that comes with a database, there are certain unavoidable performance penalties. The only way out is to figure out the bits where you don't need ACID, and use other technologies to service those bits.
