Tim over at Tim's Blog has designed what is arguably the smallest Arduino compatible board to date. The Nanite 85
I designed this board for fun after the Digispark and, subsequentally, the Adafruit Trinket were announced. The motivation was to have my own ATtiny85 based development board based on a USB bootloader and optimized for the ubiquitous 170 point mini-breadboards. In contrast to the Digispark it even sports a reset button. However, it lacks an integrated voltage converter as it is supposed to be powered by USB... -Tim
Using Micronucleus bootloader to have the Attiny85 handle the USB interface, Tim is able to keep this board super tiny. It is quite an impressive uC board layout that would work great for any USB powered project that just needs a little bit of logic and doesn't take up a lot of space. For more details, be sure to check out the rest of Tim's article. If you want to take a stab at your own tiny dev board, many of the Nanite 85 resources can be found on GitHub, including Eagle Files and code.