To dance Argentine tango is also to get accustomed to the spanish terminology and for some of us it tickles alive a wish to learn some spanish. If you plan to visit Buenos Aires your wish will rapidly grow to a plan to enroll on a course! It can happend that the only local course is run on the very same evening as your favorite milonga so it is not an option!
But there is some other possibilities available for those of us, who only have a few minutes here and there.
Duolingo - Free app
I am using this one because it has a nice layout, a good study plan and it is free. There is some more courses if you appreciate the method.
Memrise - Free app
Yesterday I came across this tool combining the good quality of Duolingo layout quite well and giving you a possibility to create your own learning materials as well. There is a lot of courses your Memrise pals have done for themselves and generously sharing for us others too. I will definetly go with this app for Asian studies and my own material!
Anki - Free app
This is quite like the Memrise app giving you the option to create your own material. I have used it for my Asian language studies and to get fluent on the functions of my camera.
All three have an app for ANDROID and APPLE as well as for the large screen on your computer at home.