Cup holder to fit regular sized take away coffee cups. I designed the clamp to fit around my stem (~24.4mm) so it's a bit too loose for standard handlebars (22.2mm) without some tape or rubber. It'd be an easy modification to make the clamp smaller.
July '14 - I found that a disposable cup would splash too much on my ride to work so I've switched to a keep cup and made the whole thing bigger to fit a medium (10oz) keep cup. The cap stop the splashing and the rubber ring keeps it from moving around too. The keep cup files are all at 1000 fold scale as I used skethup for design. Scale down to 0.001 to print.
Print in ABS (might get a bit too warm for PLA) at whatever infill you like, 40% works fine though 100% makes the clamp fairly bomb proof. Fuse the ring to the arms with ABS glue and use 12mm M5 bolts and nuts to clamp the holder to the bar.