Palo is an Multi-dimensional OLAP server developed for use with MS Excel, but still available to integrate with other applications thanks to multiple language APIs. It is described by its maker Jedox as "a cell-based database that is multidimensional, hierarchical and memory-based". Version 2.0 is available for download, but the 300 page manual, still at version 1.5, has to be ordered, and costs about 20$. Palo-Server is available under the GPL and a commercial license for proprietary developments.