The Gnusbuino is an adaptation of Michael Egger's "gnusb"  - open source usb sensor box to make it compatible with the Arduino  IDE. It has a built in bootloader and can be reprogrammed through USB from inside the Arduino IDE.
Gnusbuino vs. Arduino
- it is very simple to build, very few components and a single sided PCB
- It does not use a dedicated FTDI chip for USB connection, but the open source V-USB  library. This allows the gnusb to behave like a chameleon. It can be programmed to enumerate as any USB device and can emulate a USB DEVICE CLASS. It can be seen by the computer like a regular keyboard or joystick, if it follows the HID class - or it can behave like a USB-MIDI device.
- The gnusb can also mimick the popular USBasp AVR programmer that is supported by avrdude - so the gnusb can turn into a full fledged AVR programmer if needed
- ATMEGA644 processor clocked at 20Mhz, gives more memory and more oomph.
- it can be mounted into a Coconut to create the famous CocoGnusb :-)
- Not drop in compatible with Arduino shields
- USB serial connection is not implemented (yet?) - so no direct (debugging) feedback through serial
The Babygnusbuino is a extremely bare-bones variant based on a ATTINY85. The USB connector is directly etched on the board - you plug the board itself into your computer to upload a sketch.
The Midignusbuino is a plug-n-play USB-MIDI device that can be used to build MIDI controllers. Most operating systems have built in MIDI support, so no drivers are needed, and almost any music software understands MIDI…
The gnusb project is hosted on sourceforge: http://gnusb.sourceforge.net - but the website is currently a bit out of date (I prefer using the SGMK wiki from now on and linking here from sourceforge…) The Gnusbuino is currently a subbranch in the svn repository: http://gnusb.svn.sourceforge.net/viewvc/gnusb/branches/gnusbuino/ download the two directories "hardware" and "libraries" and add their contents to the respective directories in your Arduino directory.
Some info might also get posted from time to time on Michael's research blog http://www.anyma.ch/blogs/research