Arduino is a small prototyping board with ATmega microcontroller made by company Atmel. It was developed for school students to teach them how to build and program basic circuits, but because of its good documentation, simplicity and low price it's usage has spread among hobby developers. Nowadays it is often used as a brain of small DIY programmable devices.
Rebelix X2/S Dual
I built my own 3D printer based on open hardware RepRap printer Rebelix X2, but improved by my own design customizations. Printer consists of plastic parts made by similar 3D printer (I used 3D printers at Czech Technical University), standard aluminium profiles, stainless steel rods and bunch of nuts and screws.