A gearbox I designed, with six speeds including reverse. The ratios are 25:9, 5:3, 1:1, 3:5, 9:25, and R3:1. It's 29 studs long, so it's only suitable for one tenth scale or larger models (or the Vikings).
Rear view. The gearshift corresponds with the three forks. (The gearshift is not shown here, it's just a sliding #2 Technic connector.) The near axle (top level, center) is the output, and the far one is the input. They are separated in the center brick. The other three axles are connected through the center brick and can slide one stud in either direction.