Fritzing is an open-source graphical application for converting a project prototyped with an Arduino into a PCB schematic for manufacture. Designed for people already familiar with Processing to do what they need to do easily. Cross platform: runs on Windows, OSX, and Linux.
RepRap is an open-source (hardware, software, and documentation) desktop fabrication system which lets you construct arbitrary objects (or parts for assembling more complex systems). It's unique in that you can construct one yourself for about $400us, and it is fully capable of making duplicates of itself.
An open source DIY robotics/rapid prototyping kit much like an Erector set. Lots of standardized pre-milled bars, rods, connectors, and components. Build your own plotter, CNC machine, RepRap, scanner table, or what have you. Capable of three axes of linear motion. Comes with all the electronics you need, including an Arduino, stepper motors, and a motor driver board.