Nernst/Goldman simulator from the University of Arizona. Great for trying to get a handle on membrane potential.
Available as an iOS or web-based flash app.
Description: http://www.azps.life/home/2016/4/28/teaching-spotlight-nernstgoldman-simulator
iOS app (free): https://itunes.apple.com/us/app/nernst-goldman-equation-simulator/id1022504095?mt=8
Flash app (free, downloadable for Windows or Mac): http://www.nernstgoldman.physiology.arizona.edu/
Cheers.