BaseX is a light-weight, high-performance and scalable XML database system and XPath/XQuery processor, including full support for the W3C Update and Full Text extensions. An interactive and user-friendly GUI frontend gives you great insight into large XML data instances.
The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.
Redis is an open source, advanced key-value store. It is often referred to as a data structure server since keys can contain strings, hashes, lists, sets and sorted sets.
K. Richter, und S. Winter. Advances in Spatial and Temporal Databases, Volume 6849 von Lecture Notes in Computer Science, Springer, Berlin / Heidelberg, (2011)