Full disclosure, brat author. We were inspired by things like "What's Wrong With My NLP?" [1] and TikZ-dependency [2]. But, truth be told, there is not all that many great visualisation tools out there for NLP data. The same goes for good and freely available NLP toolkits, things like SpaCy is very much an exception rather than the rule.

[1]: https://code.google.com/p/whatswrong/

[2]: http://sourceforge.net/projects/tikz-dependency/

Brat is awesome! I just started using it last week for a project. For people who don't know what Brat is, there is a nice demo here using Brat to visualize the output of Stanford CoreNLP:

http://nlp.stanford.edu:8080/corenlp/