Another interesting category is the six-degrees-of-freedom (6DOF) controller. These aren't particularly new but, until recently, they've been targeted primarily at 3D CAD professionals and have been priced in line with relatively expensive engineering software. If you're spending thousands of dollars for a CAD package, spending a few hundred for a piece of hardware that lets you use it more easily is pretty much a no-brainer. (Devices of this type are also a good match for controlling robotics.)
However, more recently, 3Dconnexion, a wholly owned subsidiary of Logitech, has pushed down the price point considerably with its SpaceNavigator line. The SpaceNavigator PE is $59 (MSRP) for a non-commercial use license with online support and the SpaceNavigator SE is $99 (MSRP) for a commercial use license with full support. (The two differ only in licensing and support; they're otherwise physically identical and support the same software.) The company has now updated its lineup with the SpaceNavigator for Notebooks, priced at $129. It's a bit smaller than the standard SpaceNavigator and, at .55 pounds, weighs about half as much. It also includes a small case.
I've been a fan of the original SpaceNavigator for a while now. It makes a huge difference to navigating through Google Earth or Microsoft Virtual Earth. I tried out the new SpaceNavigator for Notebooks with these applications. All other things being equal, I marginally prefer the larger size and greater heft of the desktop model. However, if I were regularly using a 3D application on my notebook while traveling, the new device's design strikes me as a reasonable tradeoff for the weight and bulk savings.
The company calls the SpaceNavigator a "3D mouse" but that's a misnomer. It's only a mouse in the sense that it's roughly the same size as a mouse and you operate it with one hand. If anything, it's closer to a trackball. However, it's really its own class of input device and does not, in any case, replace a mouse except for navigation (specifically) within about 120 supported 3D applications. But it's understandable that "6DOF controller" might have been a wee too geeky for the general population.
6DOF refers to the fact that you can use the controller to generate six different motions. Pressing it front/back and left/right are the two motions that correspond to moving a mouse around the desktop. Pressing down and pulling up translate you vertically ("z" dimension for the mathematically inclined); this corresponds to altitude or zooming in Google Earth. The other three motions are those familiar to joystick users: rotation around the three perpendicular axes, i.e. yaw, pitch, and roll (or spin, tilt, and roll as 3Dconnexion calls them).
At least for me, actually using the controller feels intuitive even if it's a bit hard to explain how it works. It's a fun toy even if you don't have a serious need for one. (One hint. For Google Earth, I prefer to turn off tilt in the controller's customization panel. The tilt rotation is the one that lets you look at the surface of the earth from an angle. I typically prefer to keep the view from straight over head and, if tilt is on, it's hard not to shift it a bit while you're moving around the surface of the globe.)