I have been using the SparkFun Pro Micro. It has some distinct advantages for some projects. The other Arduino board that I like to use is the NANO. Sainsmart and others make a nice I/O expansion board for the NANO. The expansion board has a power input jack and and pins for I/O connections. What is nice the I/O pins are spread out and there is both a ground and Vcc pin by the signal pin. This board is great for working with breadboard.
It would be nice if there was a similar product for the Pro Micro. Thank you in advance for any consideration of this idea.