The MIDIbox project is an open source modular DIY framework (hardware and software) MIDI platform built around the PIC family of microcontrollers (specifically the PIC18F452, PIC16F88, PIC18F4620 and PIC18F4685) and recently with LPC1769 32-bit too. It can be used to build hardware MIDI control units for various synthesizers, multi-track recording software, and other MIDI devices; as well as stand-alone synthesizers, sequencers and other projects.
Read more about MIDIbox: History, The MIDIbox Hardware Platform (MBHP), The MIDIbox Operating System (MIOS), MIOS Hardware, Complete Solutions