Hi Rudl, it's also possible to just draw the line and look at the distance readout in the bottom toolbar (it's just underneath the x,y,z control that shows the x,y,z coordinates of the current point).

When you're drawing a line the current length is displayed there, and then push escape to cancel the command and not actually create any line, that way there's not any "waste".

There is also a plug-in that you can install which will make a new command available called CustomDistance which lets you pick 2 points and then tells you the distance but does not create any line. You can get that here:
http://kyticka.webzdarma.cz/3d/moi/#MeasureDistance

- Michael