I am pleased to announce that we are releasing a version of the reverted indexing framework as open source software! The release includes the framework and an implementation in Lucene.
Reverted indexing is an information retrieval technique for query expansion, relevance feedback, and a variety of other operations. The details are described on our web site, in several posts on this blog, and in our CIKM 2010 paper. The source code and JAR file can be downloaded from Reverted Indexing page; see the Javadocs for details of the API.