There are several very nice 3D mice out there for navigating services like Google Earth or Second Life. 3Dconnexion for example makes a whole line of devices for 3D navigation. Their compact units offer 6-8 degrees of freedom with several customizable options. The company has an SDK available and many of their devices are natively compatible with Linux (or available for access through an XInput driver). So while that is all well and great, lets look at some alternative ways people are navigating 3D spaces.