AtomSoft has designed this Arduino-compatible board that’s as slim as a DIP chip:

dipDuino Is an Arduino-Compatible Board That’s as Slim as a DIP Chip

Take up fewer rows on your breadboard with this ATmega328P-based board.

One reason the Arduino Nano became a popular form factor is that it fits onto a breadboard. The compact size is nice but makes the board a bit larger in the width dimension. To solve that issue, AtomSoftTech designed the dipDuino as an Arduino-compatible as narrow as a DIP package.

dipDuino has the same ATmega328P found in the other 8-bit Arduino boards like the Uno and Nano, however measures just 0.37 x 1.82 in (9.3 x 46.26 mm) in size. Its DIP package compatibility comes from the pin row to pin row pitch at 0.3 in (7.62mm).

The DIP-compatible form factor has all of the same pins found on those other boards: 13 digital, five analog, VCC, RESET, and GND. The silkscreen labels A4 and A5 as SDA and SCL since they have a shared function (same as the Uno and Nano).

The ATmega328P SMD packages have two extra analog inputs available that the DIP-style package does not. (This difference is why boards like the Pro Mini and Nano have A6 and A7.) Even though dipDuino has an SMD AVR chip, it does not break out the extra analog pins.