While some might argue with the value of teaching your little one to randomly mash keys on your computer, it is hard to argue with free. CrazyLittleFingers is a freeware app for Windows that locks the keyboard and blocks mouse clicks (on the primary monitor only for now) and instead of munging up your big business report, junior’s excited and curious key strokes will bring up pictures and sounds (either preloaded or user defined.)
CrazyLittleFingers via lifehacker
some alternatives:
BabySmash Also free and Windows based, but handles multiple monitors.
AlphaBaby OpenSource and Mac Based