Is This Retina? - http://isthisretina.com/
While trying to choose an external monitor to pair with a new MacBook Pro, we were left wondering about every display, "Is it retina?"
In response to this seemingly simple need, we built an online tool that calculates display density based on size, and then determines at what viewing distance the display will be considered retina-equivalent.
(We still aren't sure which monitor to pair with that MacBook Pro though.)