I have a space navigator that I use to navigate round in sketchup and it's great, I paid £20-£25 for one a few years ago but they seem to have shot up in price to £80-£100. Not sure if I would pay that much for one, but if anyone is finding mouse navigation in 3D a pita and also has a chunck of change burning a hole in their pocket...

This is a demo of one being used in eaglecad

Use a 3D mouse with Eagle CAD

And if the price is offensive to you you could always make your own with a cnc machine/3d printer

Futuristic 3D mouse originally Arduino powered and 3D printed