In this paper we describe XXL (eXtensible and fleXible Library), a toolkit for rapid prototyping query processing algorithms. XXL is a flexible, high-level, platform independent Java-library. XXL provides a powerful collection of easy-to-use index-structures, query operators and algorithms facilitating the performance evaluation of new query processing developments. Query algorithms in XXL use the same set of basic classes, like I/O routines and improved (de)serialization methods. Therefore, XXL is an ideal testbed for experimental comparisons of new approaches to established ones. As examples how XXL can be used for implementing and comparing algorithms, we provide different state-of-the-art spatial join algorithms and several new indexing techniques in XXL.