In concert video footage of Muse, I've seen both the guitarist and bassist use touch-pad devices integrated with their instruments.
What are these devices, and what are they designed to do?
Those are x-y MIDI controllers more commonly referred to as Kaoss Pads.
If you are familiar with a modulation wheel on a MIDI keyboard, you know you can assign that wheel to control any number of aspects of the instrument, from pitch bend, to volume, to vibrato, an LFO, or perhaps a filter. The x-y pad gives you two of those on a plane, with which you can control anything that's MIDI controllable (even lights if you've got the right setup).