USB does fine for a microcontroller power development platform. But what do you do when you want to let some project loose in the wild? That is one of features Itead includes in its Sonoff line. For about the price of an ESP8266 development board (~ $5) you can get an ESP8266 with 1 MB SPI RAM, an LED, a pushbutton, a relay, and a power supply that taps off the 90 to 240 VAC that the device is intended to control all in a nice box. In addition, you get firmware on the chip, a free Android or Apple app, and a service to connect between the chip and the app for control and status. That means you can easily get up and running to see what is possible. Then you can look at doing your own firmware, control network. and software.
Itead says
Our goal is to fuel an innovation revolution with easy to use prototyping modules, low cost development platforms, and even custom made solutions to help you get your great ideas in to fruition in the quickest possible time.
Towards this end, they have made it easy to add a header to their boards so you can attach a serial to USB adapter and upload your own firmware. There are several projects online that require just a small step in difficulty from the Itead supplied system. ESPEasy puts a web page on your Sonoff to configure your ESP8266 device including working with some of the more popular home automation systems. Sonoff-Tasmota needs configuration in its source code but is more focused on Sonoff and MQTT. Others include ESPurna and KmanSonoff, and Jonathan Oxer has Six Sonoff Secrets, a blog post with a few more ideas including upgrading the flash RAM.
You can also run micropython on these things. That is rather cramped with the standard memory chip but the code for many Itead accessories and other peripherals you might want to use for an IoT device are readily available.
It’s not much to invest in either money or time or learning to find out what the Internet of Things is all about.