Lucene is an Open Source, mature and high-performance Java search engine. It is highly flexible, and scalable from hundreds to millions of documents.

Luke is a handy development and diagnostic tool, which accesses already existing Lucene indexes and allows you to display and modify their content in several ways:
browse by document number, or by term
view documents / copy to clipboard
retrieve a ranked list of most frequent terms
execute a search, and browse the results
analyze search results
selectively delete documents from the index
reconstruct the original document fields, edit them and re-insert to the index
optimize indexes
and much more...
Recent versions of Luke are also extensible through plugins and scripting.