4PocketsAudio has a 50% off sale on their Syngery Studio app! This brings the price down to just $10.
Synergy Studio is a sequencer that enables music creation by all musical abilities. By simply tapping on the grid interface users can lay down a series of notes and patterns of up to 64 notes to create a musical sequence.
If you are a seasoned musician looking for a tool to help lay down new ideas or need more advanced features then Synergy has the lot, a fully integrated mixer, lots of plug-in effects, complete customization of instruments and patches and extensive automation facilities.
There are four main instrument categories to choose from: Drums, Analogue Synthesizer, Sample and Pad Synth. Each category contains different types of instruments - for example in the drum kit category you can choose from an array of different kits including Analogue T9, Orchestra, Rock and Techno. Further instrument packs are available in the Synergy’s free sound library or as In-App purchases.
Synergy includes MIDI implementation enabling you to interact with MIDI hardware, such as Synths and Samplers creating a powerful live tool.
I haven't played with this app, but it looks fairly interesting in this detailed tutorial.