Friend of mine beat me to it... check out the post Kill Your Database with Terracotta over at Will Code 4 Beer.
It is an interesting article challenging the idea that we need a database to begin with. There may be an excuse for a database in some cases, but for many applications it is overkill. When you need a super fast, scalable application to hold on to just enough data then Terracotta can store your objects to disk and allow your entire cluster to share state through the shared Heap. It is an interesting read, so go check it out.