I am currently developing DSerial which is going to be a debug cable and more. It has an integrated 8051 microcontroller that can be programmed for specific applications. USB, serial and I/O ports allow using this device for a range of ideas. An obvious application would be a debug console and perhaps even GDB support. Serial port allows to connect GPS receivers, motor controllers and other gadgets equipped with UART or RS-232 serial port. 18 I/O lines enable electronics enthusiasts to connect various projects that could use the power of two ARM CPUs. Analog-to-Digital Converter (ADC) is useful for a range of sensors and simple oscilloscope projects. There’s also a 2D tilt sensor (optional) that could be used in homebrew games like Marble Madness clones (please make this ).
I haven’t tested the hardware yet, but below are some pictures of the current state.