The Duet was developed by Andy and Tony from Think3dPrint3d in conjunction with RepRapPro and with much advice from Chris Palmer (Nophead)
The Duet is a 3D Printer controller board that is compatible with the Arduino Due. This 3D Printer controller combines the Arduino Due microcontroller with 4 stepper motor controllers, Ethernet, Hi-Speed SD card slot and more.
The Duet runs the 32 bit, ARM core SAM3X8E microprocessor, as found on the Arduino Due. This is a step change from preceding controllers using 8 bit microprocessors and leaves loads of overhead to do cool things (like running a webserver or running delta bots much faster). The Duet comes with Ethernet built in allowing it to run as a network printer controller, along with USB and an SD card (fully SD 2.0 compliant). In addition it takes the Duex4 expansion board for a further 4 stepper channels - for a total of 8 axis controls (3 axis + 5 extruders for example). A more in-depth description can be found in this blog post
The Duet hardware design is licensed under the CERN OHW License 1.2: the design is free to be distributed and modified within the terms of this license. All the design files are available on Github. The Duet was completely designed using the Open Source software package KiCAD so hacking and building on this design its accessible to all.
The Duet runs RepRap Firmware, a new C++ firmware by Adrian Bowyer. The software supports receiving GCode from over the USB serial port, from the SD card and from the ethernet interface. Additionally, the firmware can be compiled with the Arduino IDE or Eclipse and uploaded like other firmware.
The software is adding new features daily, for example support for multiple extruder printing - see the T3P3 github, RepRapFirmware, multi extruder branch.
Here is a picture of a 4 colour print from a Duet and Duex expansion board combo.