Over a year in the making, Instructables user Kyle aka Shadowman39 made this skeeball machine entirely from K’Nex parts, even the coin slot and ball return mechanism. Very impressive!
This machine is full-size, coin-operated (takes quarters), turns itself off automatically, and counts points using a mechanical counter. And, all of it is mechanical, using only normal motors to power it (no CyberKnex motors). There are six motors used in the machine; four of them are used for just the counter. The only non-Knex parts used are a few rubber bands and the paper for the counter.