The storage mechanism behind Zope. Can be used on its own as well.
RichardJones said if you use the ZODB to store a large dataset that wants to be relational, or needs to be queried / accessed like a relational table, you'll find that your data is pretty much duplicated (or triplicated if index and meta-data) inside Z Catalog-s to have any sort of speed. Just use an RDBMS in this instance. Well, unless your data schema is highly mutable (as ours is) in which case just accept that half your ZODB will be Z Catalogs.