FreeSensors have a set of shields that connects withs sensors using standard RJ11/RJ12 modular connectors, differencing the type of connector with colors (for example: blue is for analog inputs, gray for digital input/output, etc.). The idea behind is to achieve a modular system that could be applied to educational robotics for childs.
The simplest shield is Direct with 4 analog input ports and 4 digital I/O ports directly connected to the Arduino UNO pins. The DIOport is an expansion board with 8 digital I/O ports thanks to the PCF8574 chip with I2C communication. Based on that shield is the Motor one, with connectors for 2 DC motors, 2 servos and 4 digital I/O ports. The AIport shield uses the CD4051 chip to multiplex 8 analog inputs over one analog input of Arduino. Also, some other shields are being designed. All the shields are true stackable because all the pins are passed through, and a compatibility matrix can be found in the site.
A library was developed for accesing the shields, to abstract the inner working of the shields and in a way that, for example, the digital I/O ports are accesed equal no matter what shield they are.
Sorry, but the site is in spanish. Hopefully, soon I will traduce some pages to english. All the shields are open-hardware and the Arduino library is free-software.
Comments are welcome,
Best regards, Mario