The littleBits library is the easiest electronic construction kit around! Conceived of at the MIT Media Lab, littleBits makes electronic modules with specific functions such as lights, motion and sound. Simply snap them together with magnets to use them for prototyping, learning, and fun. littleBits has won over 20 toy awards on its path to demystify electronics and spark the world's next generation of inventors!
Check out the Littlebits project page for more project ideas.