PT Sans is another great typeface for body copy that we can use both in apps and websites. Of course it’s also free which is always nice and it’s not helvetica+ ;) I suggest you go ahead and download it from http://www.fontsquirrel.com/fonts/PT-Sans right now.
I’m especially happy to see that we’re not limited to Helvetica Neue, Roboto and Segoe for our mobile designs with great free fonts like this one, Open Sans or Source Sans Pro around :)