Inclusion in this list does not imply an endorsement or personal recommendation.
Some of these can be used by individuals, others need to be set up as classes.
Some are free; others are paid.
http://www.lingrolearning.com/ -- Scroll down to where it says "Lingro To Go" (free app)
http://beelinguapp.com/ (Side-by-side dual language reading app, with audio )
https://www.verbalplanet.com/ (lessons with tutors via Skype)
https://mangolanguages.com/ (paid only; although, some libraries have a subscription to it)
https://lingopie.com/ (paid only; learn through TV shows)
Lingro - Make every word on web pages clickable so you can see the translation, just by entering the URL. Similar functionality to the Google Translate extension but without installing anything. It also remembers the words you looked up so you can create word lists, and play games.
Forvo - Pronunciation dictionary
Hi Native - Ask native speakers questions (similar to the Wordreference forums, but as an app)
Yask - As questions and get feedback from speakers of the language (similar to Hi Native, but in a "game" format)
Lang-8 - Platform where native speakers give you feedback on your writing