Adafruit has several Arduino compatibles: The Menta, DC Boarduino and USB Boarduino. Each has certain design features that make them more suitable for some purposes:
The Boarduinos are designed to plug directly into a breadboard to simplify breadboard prototyping. They can also be used as a compact, low-cost controller for embedded projects.
The Menta has a large prototyping area on board. This makes it easy to build more permanent custom circuits right on the board. And the whole thing fits perfectly into a mint tin for a compact, clean package.
All these boards are software compatible with standard Arduinos.