Logitech Trackman Marble (formerly aka Marble Mouse). But skip the OEM config/driver software and just use
X-Mouse Button Control (Windows) or
USB Overdrive (Mac) instead, much more flexible button-assignment options that way.
When using this trackball, you just rest the pinky edge of your hand on your desk alongside it and then "drape" your hand over it. It's a fairly neutral wrist position, though you may want to put it on top of a cushy mousepad for comfort. It's also symmetrical, so you can switch hands if one gets tired; I use X-Mouse Button's layers feature to define mirror-image left-handed and right-handed button assignments and switch between them easily. An issue with thumb balls is that they're always asymmetrical and thus almost always right-handed.
As an aside, the old Kensington TurboBall was a similar design but IMO even more ergonomic, as the body was taller and the ball slightly larger, allowing a hand draped over it to remain more vertical, and it also had a scroll wheel along with 4 larger buttons that could be "chorded" (2 buttons clicked together could have their own button assignment); however, it was let down by tiny mechanical ball-tracking rollers that constantly got gunked up and needed frequent cleaning, as well as somewhat fragile internal construction. If Kensington ever re-released an updated TurboBall with optical tracking and maybe a scroll ring around the ball, it would be well-nigh perfect.
