Layout

ObjectDistance / CameraDistance / LightDistance

This part of RealiTools is a handy little set of utilities that will do distance measuring. They can be found in Layout, under the Options screen, as Generic plug-ins. They are all very similar, so if you know how one works, the rest won't give you any surprises. First, open the Options panel in Layout. Switch to the General Options tab, if necessary, and open the Generic Plug-ins drop-down list. Then, choose the type of measuring you want:

RealiTools_CameraDistance: Measures the distance between an object in the scene and the camera.
RealiTools_LightDistance: Measures the distance between an object in the scene and a light in the scene.
RealiTools_ObjectDistance: Measures the distance between two objects in the scene.

When you choose one of these plug-ins, a new window will appear on the screen. In ObjectDistance and LightDistance, there are two drop-down lists at the top of the window (there is only one in CameraDistance). Using these, choose which items in the scene to measure. The slider bar below will go through all the frames in the scene. As you slide it, the frame number (and the values below) will change. The first three number boxes (the "Delta" values) are the differences in X, Y, and Z positions of the two items in question. The info in parenthesis shows the order of subtraction. Finally, the last number box, "Absolute distance", is the ol' straight line between two points. Click OK to close the panel.
CameraDistance can come in really handy if you ever need to figure out fog or depth-of-field distances. The LightDistance plug-in can be useful when you're trying to get surfaces lit correctly. You should bear in mind that the distances measured are between the pivot points of the two objects. Finally, as an added convenience, you can click inside the number boxes to copy the numbers. Changing them won't do anything, though.