We've been working hard over at MakingThings on a new product we recently released in collaboration with Make Magazine - the Make Controller Kit.
The Make Controller Kit is a general purpose, high performance microcontroller with all sorts of nice features - 10-bit analog inputs, 1A digital outputs for DC and stepper motor control, servo motor control, Ethernet, USB, RS232, SPI, CAN, and JTAG among others.
Great for interfacing all different kinds of sensors and actuators to your PC. All the firmware, and software tools for the board are open source and freely available - schematics are open as well.
The board can be easily reprogrammed, but by default runs a program that speaks OSC to a connected PC, over Ethernet (UDP) and USB. In Max, CNMAT's OpenSoundControl and osc-route objects work very nicely over the network, and we've just put together some USB-based objects as well.