http://www.cs.cornell.edu/home/llee/data/
http://mpqa.cs.pitt.edu/corpora/mpqa_corpus
You can use twitter, with its smileys, like this: http://web.archive.org/web/20111119181304/http://deepthoughtinc.com/wp-content/uploads/2011/01/Twitter-as-a-Corpus-for-Sentiment-Analysis-and-Opinion-Mining.pdf
Hope that gets you started. There's more in the literature, if you're interested in specific subtasks like negation, sentiment scope, etc.
To get a focus on companies, you might pair a method with topic detection, or cheaply just a lot of mentions of a given company. Or you could get your data annotated by Mechanical Turkers.