Apache Lucene - High-performance, full-featured text search engine library written entirely in Java.
Figure - Propellerhead creates world-class software products and services that inspire music makers and provide the foundation for a worldwide creative musical community.
Six Degrees of Wikipedia - Find the shortest path between two subjects
SoundGrail - SoundGrail Music App designed for pianists, guitarists, and DJs.
Sphinx Search - Sphinx is an open source full text search server, designed with performance, relevance (search quality), and integration simplicity in mind. Sphinx lets you either batch index and search data stored in files, an SQL database, NoSQL storage.
Splice - Music creation, collaboration, and sharing made simple.