I love Arduino. First because has be build with open source in mind and everything related to it is completly free. So if yu want to build your own, or adapt it to your needs, just download CAD files, modify them and build it. Another thing what I love to Arduino it is the plug an play. Just atach a battery and connect it to your computer (USB) and you are ready to program it.
Arduino is build around ATmega328 with a 16MHz crystal osiclator and it has 14 digital input/output pins
|Arduino Uno (Revision 3)||£18.98|
|Arduino Uno Board REV 3||EUR 17,99|