Finally, Duolingo added a En->ZH course. As Duolingo is the most popular gamification learning app, the learning experience is relatively streamlined and tested. The monetarization aspect leads to some battle against the system though ("Health", "Power Ups", etc.) as well as to have watching ads, but it can be used without paying.

Also has a intro test to skip some lessons, which is fine for expert beginners :D

There is also a Chrome Addon that can switch between Traditional/Simplified characters: https://chrome.google.com/webstore/detail/duolingo-chinese-word-def/clnnjdojceobnkhkocigpboialomopfk/related