This applet simulates the Hodgkin-Huxley Nerve Model, much like the first HH applet. But here one can generate a phase space plot for the variables by clicking on the "phase space" button. In addition, one can change the time for the two stimuli as well as their duration and current amount.
The model can be found in J. Physiol. Vol 117 500-540, 1952 by A.L. Hodgkin and A.F. Huxley.