Who are major competitors to Solr?
My answer to Who are major competitors to Solr? on Quora
ElasticSearch is a really interesting one—it’s the same underlying search library (Lucene) and the same integration model (an HTTP interface) but takes quite a different approach. It hasn’t been around for a long time but it looks very impressive: http://www.elasticsearch.com/
Other than that, popular open source search engines include Sphinx and Xapian. I’m a big fan of talking to a search engine via HTTP, so I’ve been keeping an eye on the http://www.flax.co.uk/ project which does that for Xapian.