About two years ago, for no really good reason, I designed a 3D-printable setup for coupling those standard gearmotors to Lego bricks and axles, and posted it at Youmagine. Hardly anyone liked it or downloaded it.
Then, last week, Adafruit, arbiters of all things cool, highlighted a cute new product: a little injection-molded piece that connects standard gearmotors to Lego wheels and gears. Essentially my stupid little adaptor piece plus an axle built in. So now I feel that my original idea was as blindingly obvious as it seemed to me. I was just ahead of my time and/or my implementation was crap.
Seeing their solution also made me think a little about design for manufacturing. My little cylinder was optimized for 3D printing, where leaving space out is easy and making long, skinny, precise and strong bars, beams or axles is a pain. Their socket-plus-axle is just right for injection molding, where molds for positive shapes is easy but making inserts for precise, deep straight holes is harder.
Oh, yeah, I still have those old RCX bricks lying around.