Molten Drum Machine (link to mini-review) has been the drum app I recommend most. It was the first to offer both Audio Copy/Paste (for sample importing) and Virtual MIDI. In fact, as far as I know, it is still the only dedicated groove app that has both of these features. In many ways it felt more like the hardware Electribes than the iElectribe does. One Red Dog has decided to focus attention on the future of Arctic Keys and can no longer support Molten. Before it is removed from iTunes, it has been made available for free this month!
Grab it while you can, and make sure you back it up on iTunes... but please don't bug Peter for support! In memoriam, here is the original trailer released with it back in November 2010.