A flexible measuring tape (think the kind tailors use) marked off, not in inches or centimeters, but in inches or centimeters times pi. The back side has inches or centimeters divided by pi.

Allows one to quicky and easily determine the radius and diameter of a circular object by measuring around
the edges, or, using the back side, the circumference by measuring across. No calculations necessary.

(Granted, the occasions on which you would need to do this are likely to be rare.)

//elativity-corrected tape measure for measuring things
which are travelling at near-light speeds.//

An aside: given that things change size when they approach
light-speed, and given the principle of microreversibility in
physics, it should be possible to make a rubber spaceship,
and propel it to light-speed by simply squashing it (or do I
mean stretching?).

[normzone], thanks for the link - I was imagining something a lot less precision and calibrated, just a thingy one could pitch in one's toolbox next to the ikea hammer and the screwdriver with sixteen bits, five of which have even been used, but those are very cool.

[+] Do the two sets of numbers need to be on opposite *surfaces* of the tape? Surely using the two edges of one face would suffice, and would cut down on printing costs. This is commonly done for tape measures and rulers which show both metric and inches, so I don't see why it wouldn't work for conversions between circumference and diameter.