The company Bug Labs is building a modular set of hardware devices which can be plugged in to each other to create a larger set with a number of features. However, I don’t think this is where the future lies.
Instead, what is needed is a generic, open protocol for transparent interconnection and data transfer between various hardware devices, regardless of the manufacturer or the included modules. The best way to achieve that is wireless communication, so the only required module should be a WiFi controller.
With this, we could connect our iPad to our microwave oven, or our car alarm to our Twitter account, and with the right software they would communicate perfectly, enhancing each other.