The folks at CERN wanted a better way to search High Energy Physics fulltext paper repositories and bibliographical databases that produce result set numbers in the multi-millions. INSPIRE, the that merges the sources' query results, though, is written in Python. In order to move back and forth as quickly as possible between the two systems, CERN decided among a number of options to embed INSPIRE in Solr.
The result, MontySolr, utilizes the power of Java and works with any Python applicatio, as well as any C/C++ app that Python understands. For more information on MontySolr, check this video of Roman Chyla (CERN).