haplome is an open source multimedia interface for the iPhone and iPad. It is based on the monome grid and is compatible with applications designed for use with a monome grid. You can browse the list of monome applications if you would like to get an idea of what is possible.
Description of common uses from monome.org:
live sample cutters, sequencers, math simulations, drum machines, generative controllers, tonal maps, games, visualizations.
Description of common uses from wikipedia:
A monome is a real-time step sequencer made up of a grid of backlit buttons that can be utilized for a number of applications, the most common of which is music performance. They are often used to trigger and retrigger samples or sample sets, but can also be used as a generative instrument that runs self-effecting or self-sufficient patterns, or to control effects and envelopes.
The source of haplome is available on GitHub. If you would like to add features or fix any bugs, please feel free to fork the repository. haplome has been removed from the iOS app store because my Apple Developer account expired, but you should still be able to install it from source.