While picking a motor for a tethered electric airplane project I do with my junior students, I discovered that Mabuchi has some very interesting interactive torque curves for their motors
at their web site.
Just search for a motor of a particular spec... I was looking for a diamter of 22-25mm, and you'll get a list of motors. The orange "sim" dot starts a shockwave window that lets you play with voltage and read the figures right off the chart. I'll probably use it with students when discussing motor power/torque/current curves as I haven't seen anything else on line that shows the effect of increased voltage on motor performance in nearly such a friendly way.
Jason