Is There a way to search non drupal solr index? I have a huge solr index. I'd like to build my UI within a Drupal site. I tested the apachesolr module but it only focuses on a Drupal node. Does anyone have a similar project?
The Sarnia module is the only solution I'm aware of. It leverages the Search API, but unlike the other SOLR modules, it is designed to expose the content of any index rather than index Drupal content. It also has some pretty decent documentation, here.

However, I'm having trouble getting it to actually work properly. Latest dev version is not working for me, at least at the moment.

[UPDATED] Most of my troubles arose because my SOLR index does not use the standard handler names, and so some manual configuration was required (hacking service.inc). Once done, it certainly works, although it does not cope with grouping/collapsing results, and I'm not sure how to expose facets.