Paintball guns: spool valve vs poppet valve
Rube Goldberg device (full-screen version — it’s modular — complete with annoying music)
A toy I’ve seen. Not sure if/where this is incorporated in functioning machines.
Radial engine and Wankel rotary engine (more engines at a site to which I’ve previously linked)
Geroter aka “Generated Rotor”
You forgot the world’s most useless machine! 🙂 http://www.youtube.com/watch?v=Z86V_ICUCD4
“A toy” is also known as a “bullshit grinder.” The handle moves in an elliptical motion, so it has utility for drawing ellipses, perhaps for feeding work into machines.
It’s not so much “forgot” as “I compiled these on my lunch break, and youtube gets filtered at work.” So I was limited to animated gifs.