Alexander Refsum Jensenius has pointed me toward the gluiph:
The gluiph is a single-board computer that was conceived as a platform for integrated electronic musical instruments. The gluiph aims to provide new instruments as well as existing ones with a stronger identity by untethering them from the often lab-like stage setups built around general purpose computers. The gluiph’s key additions to the standard core (CPU storage etc.) are a flexible sensor subsystem and multi-channel audio I/O. The gluiph in contrast to other stand-alone approaches retains a higher degree of flexibility by supporting popular music programming languages with Miller Puckette’s pd being the current focus.
At his weblog Tim Place is linking to uCApps a resource for free MIDI do it yourself projects.