I have heard that the Duolingo app is a really good tool for learning languages on the move. However, it was first released only for iOS devices (iPhone, iPod touch, etc.) but recently it has now been made available to Android users.
My Android phone (Samsung Galaxy Ace) is relatively old, and runs the 2.3.4 operating system. The majority of new apps do not work on my phone (for example, the Memrise app). I was expecting the usual incompatibility issues, but to my surprise it runs perfectly!