STM32F4Discovery board is already quite powerful and stacked with many handy features. On a single board, you can find a three-axis accelerometer, MEMS microphone, DAC with D class amplifier that can output sound through the built-in audio jack. Also, there are a couple of LEDs and a button for fast access. And of course, the core of this board is STM32F407VGT6 ARM Cortex-M4 microcontroller that packs series of great features including DSP instructions and floating point unit. But eventually, with such power, you will miss several other handy features that could be used in your project. Think of LCD, Ethernet, RS232 interface and even camera. You could start making your extension boards that include additional features, but why bother, since there already are several extension boards available for it. Let us go through them and see the features you could get without a heating soldering iron.