Blacklight is a free and open source ruby-on-rails based discovery interface (a.k.a. “next-generation catalog”) especially optimized for heterogeneous collections. You can use it as a library catalog, as a front end for a digital repository, or as a single-search interface to aggregate digital content that would otherwise be siloed.
Blacklight uses Solr, an enterprise-scale index for its search engine. Blacklight features faceted browsing, relevance based searching (with the ability to locally control the relevancy algorithms), bookmarkable items, permanent URLs for every item, user tagging of items.