RhombusLib is an Android library project that decodes magnetic stripe audio data.  It is designed for use with the unencrypted SquareUp credit card reading dongle, though it should work with any mechanism to get the raw data from the card as audio.
RhombusLib can decode either track 1 or track 2 data, and can even determine on the fly which one to decode to.  It supports swiping in either direction.
RhombusLib is not an app.   It is a library to use in other apps.  For a demonstration, see the Rhombus app in the android market (google play).  https://play.google.com/store/apps/details?id=me.cosmodro.app.rhombus

RhombusLib is licensed under the GPLv2.  Contact steve@cosmodro.me for commercial licensing.