ToneCheck Checks Your Tone Like A Spell Checker Checks Your Spelling

This is a fairly well studied area, search on the keyword "sentiment analysis" for more information, including a number of open source implementations. ToneCheck even seems to be using the 6 basic emotion categories that everyone else uses, although they've tweaked the names a bit.

It looks like they've crowd sourced evaluating the emotional content of words and phrases at http://toneaday.com/.

My guess, largely without evidence, is that they are primarily basing their analysis on words and phrases and are relying on a better word->emotion dictionary than most to provide a competitive advantage. There are a number of open source tools that do this sort of thing, the ones I've seen relied heavily on the dictionary approach as well.

This is a very clever idea and positioning, certainly a better way to productize this technology than the ideas that had been floating around my head.