The BONSIA and SPHERES free modeling tools are live and FREE. The most advanced AND simple to use freely available modeling packages to help design TES montages including for tDCS, tACS, tRNS, and more. Check them out and give us your feedback on how to improve!
Read the PDF of the epub: BONSAI_bikson_2014
BONSAI is online here
SPHERES is online here
The paper explaining the principles is here