The RepRap project (http://reprap.org) has just chosen the Arduino for its future developments.
RepRap is a 3D printer that makes real solid plastic objects, so you can use it to make lots of useful stuff.
RepRap can also print most of itself, so - if you have one - you can print another and give it to a friend. That is to say, RepRap is a self-replicating machine...
Following the principles of the Free Software Movement, RepRap is being distributed at no cost to everyone under the GNU General Public Licence.
Best wishes
Adrian
Dr Adrian Bowyer
http://staff.bath.ac.uk/ensab
http://reprap.org