The problem with modular nowadays is that it is so darned expensive. Not so miniMO, an open source and open hardware modular solution developed by Jose Gonzalez.
The idea behind the project was to create a modular synthesizer that was as affordable as possible.
Two editions are available; a plain PCB-based version and a 3D-printed version. With the latter, everything is printed bar the components. You’ll also be pleased to know that the breadboard is printed, so no need for any soldering.
All the modules are identical, but can be programmed (via the Arduino IDE) as an oscillator, a filter, or a sequencer, and Jose is already developing more blocks
Want to know how you can build your own? Check out this step-by-step guide.
For more information, go to Jose’s project page. You can source all the components and schematic required from github and thingiverse.