Arduino is most active and successful open hardware project. One of major reason is simple hardware and simple language to program with flexible and easy-to-use modules.
Person having basic electronics knowledge can start with Arduino in an hour or so. Furthermore, Availability and price of Arduino boards is major advantage. Plenty of country has local vendor (including some having multiple vendors.) The add-on card such as blue tooth shield or Wi-Fi shield makes it more usable. Arduino forum and wiki are best place to get any help related to Arduino.
Based on exact purpose of project, you can select best Arduino hardware. Ethernet, Xbee and motor shield can be quite useful for experiment. Typical Arduino core has ATmega series chip-set.

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