Cassandra has the features you are looking for, including adding nodes as needed, and is starting to get a bunch of success stories outside of its original auth

Cassandra has the features you are looking for, including adding nodes as needed, and is starting to get a bunch of success stories outside of its original authors (facebook). Digg, for instance, has 9TB across 12 machines. They wrote some articles about it at http://arin.me/code/wtf-is-a-supercolumn-cassandra-data-model http://arin.me/code/wtf-is-a-supercolumn-cassandra-data-mode... and http://blog.digg.com/?p=966 http://blog.digg.com/?p=966. (I work for rackspace and we have an internal app going live on Cassandra in the coming days. But it doesn't technically qualify for "in production" yet.)