I found DuoLingo interesting, but it didn't really help me to learn Spanish. I had a lot more success (I just came back from a trip to Panama where I had to interact with people who spoke little no no English) using https://www.languagetransfer.org (it also has courses for Greek, French, German, Arabic, Turkish, Italian and Swahili).

The courses are sets of MP3 tracks that are free to download, but if you find them useful, please donate!