Thanks to the Solr team for inviting me as a committer!
Solr adds clean RESTish interfaces and strong "enterprise features" to Lucene, and as such is a very useful tool to integrate search functionality in web applications. It makes the power of Lucene very easy to use (see the tutorial and my article for more info).
I discovered Solr earlier this year when teaching my "web lab" class at a local engineering school, and later this year evaluated it for an important customer project, that should go online soon. You'll hear more about it!