Arduino ISP programmer
There are lots of things you can build with an Arduino. The prerequisite, though, is being able to program the microcontroller sitting on the Arduino — that is, to put into words the computer understands what the microcontroller should do, and to store the result on the microcontroller. Programming microcontrollers is, surprisingly, a task you can carry out perfectly well with an Arduino. Some enthusiasts have already looked into the problem and written Arduino programs for programming microcontrollers. Here are a few examples: