K'nex builder Austron recently completed the largest K'nex ball machine as an installation at The Works interactive museum in Bloomington, Minnesota. from the YouTube page:
The machine stands 23.5 feet tall and 40 feet long, and contains over 100,000 pieces. It has 2 lifts, 3 motors, and 8 paths, including a 20 foot free-fall, an 8 foot tall big-ball-factory spiral, and a 60 foot long path which hangs from the ceiling. It takes 3 and a half minutes for a ball to climb to the top of the tallest tower.
-via Viral Viral Videos