Lucene is at the heart of open source search: the index is at the heart of Lucene.

Lucene/Solr index implementation is critical to the performance of your search application and the quality of your results — and not just at indexing time. If you're developing applications in Lucene/Solr, your index will reward care and attention — adding power to your running search application — all the more so as you inevitably increase the scope of your query traffic and the dimensions of your data.