Stefan Feuerriegel and Nicolas Pröllochs have a new package in CRAN:
Our package “SentimentAnalysis” performs a sentiment analysis of textual contents in R. This implementation utilizes various existing dictionaries, such as QDAP or Loughran-McDonald. Furthermore, it can also create customized dictionaries. The latter uses LASSO regularization as a statistical approach to select relevant terms based on an exogenous response variable.
I’m not sure how it stacks up to external services, but it’s another option available to us.