The Smallest 3D Maker won't be the fanciest online drawing tool you've ever used. However, with a total size of less than 10KB, it may well be the smallest.
Written as part of a competition to build compact webapps, the app converts Bézier curves into 3D wireframes which you rotate on screen. It's simple to use, but quite addictive, and a good way of learning how 3D objects are constructed. While it should work in any browser, it relies on the keyboard for controls, so tablet users can't join in the fun. Obviously with that size there's also no sophisticated export options, but there's always screen capture for the basic image.