Chordbot Pro (Android) app for $2.99

Chordbot is an auto-accompaniment app for Android that lets you create and play advanced chord progressions easily. Use it for songwriting experiments, as accompaniment for your guitar/saxophone/theremin solos, or as a backing band for your shower singing sessions.

Featuring 60 chord types and over 20 different comping styles, this app provides you with a wide range of musical backing that's designed to be simple but effective: No gratuitous brass fills, just unobtrusive patterns that won't get in the way of your compositions. Chordbot's styles aren't based on prerecorded loops--they work as dynamic sequencer templates that are used to synthesize your songs in real-time. The songs are then played back with instrument samples from the GeneralUser SoundFont bank.++