My goal is to build a review site from analyzing social media text, I started with the web hosting industry. You can take a look at the results here: http://reviewsignal.com
http://theysay.io/ -- These are Oxford-based researchers with a pretty thorough text-mining API. Less about indexing and more about text analysis.
not sure if they are the best, but I've read about blinkist a while back (http://www.blinkist.com/en/). apparently their app uses some smart nlp magic to give people summaries of non-fictional texts/books.