Retronyms has Tabletop (iTunes Link) for the iPad in the iTunes App Store for 99-cents for a limited time.

An Expandable Studio
The heart of Tabletop is a modular environment where you can mix and match different devices. Select from more than a dozen devices including samplers, mixers, effects, sequencers and more. Buy only the gear you need from an expanding catalog.

Tabletop is the first musical environment designed from the ground up exclusively for the iPad. Each device has its own uses, characteristics, personality—ranging from classics like drum pad samplers to touchscreen effects.

Virtual, Modular Gear
Plug the output of a keyboard into a delay to add an echo, or wire it up to a mixer and adjust the pan. Chain up multiple effects to create new and evolving sounds. Tabletop is an audio playground where you can create beats, compose songs, mix live, make mashups with samples from your iPod library… the possibilities are wide open.