How funny. I discovered this one yesterday as I was trying to read the amazing 9to5Mac story (http://9to5mac.com/2014/08/29/se...) and started searching for a speed reading tool. Works well! Only gripe with it is that I wish it would do some sort of overlay on the rest of the page when it pops in.
I recommend also checking out http://beelinereader.com as an alternative for making reading faster and easier, without many of the downsides of Spritz.
@tectonic BeeLine looks like a very different product, but also interesting to see their take on solving the same problem. (BTW, you probably should have disclosed that you're CTO of said company.)
I discovered Spritz as a desperate slow-reader starting college 4 years ago and it has been a life saver. The bookmarklet (or as they call it, "Spritzlet") has made it way easier for me to long articles in my browser, which has actually gotten me to read more than I would have. I also recommend http://www.readsy.co/ if you want to use Spritz to read a PDF or text you can paste in that couldn't be read on its original web page.
Product Hunt