The AVR.duino U+ board from SlicMicro has more development-board-like features than the Arduino Uno R3. It has an additional LED, potentiometer, and pushbutton, all exposed and available. It also has the addition of A6 and A7, which is great if you don't plan on porting back "down" to an Arduino Uno. And finally, it has a "SlicBus" port, used to interface to SlicMicro's module system.
AVR.duino U+ also includes a crystal oscillator instead of the Arduino Uno's ceramic resonator, for more precise timing. It has 8 analog inputs, instead of the typical 6. A clever addition is the bottom side silk screen, which shows the ATmega328 pin designations next to the Arduino pin designations.
It's cousin, the PIC.duino Net, has the same SlicBus, an ethernet port, and runs on a PIC18F67J60 MCU.
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