There's several companies working on sentiment analysis. The NYT had a roundup: http://www.nytimes.com/2009/08/24/technology/internet/24emot...

Backtype is a YC company that probably does some NLP stuff on the backend. http://www.backtype.com/

Also, Powerset and Wolfram Alpha.

The most obvious killer app is sentiment analysis applied to product reviews. Overall I don't feel like NLP has game-changing applications yet since good data is structured data. The research is in place, though.