This one's a neat one: Pickett N909-ES SIMPLEX TRIG RULE with METRIC CONVERSION. It comes with a set of gauge points on the slider that lets you convert between inches/cm, pounds/kg and all that. It also has a Fahrenheit/Celcius scale and a novel scale for switching from decimal hundredths to fractional 64ths.
Unfortunately, this rule wasn't intended for Canadians -- the temperature scale only goes down to -25 C.
(The layout of the scales really drives home the point that multiplication on a logarithmic scale is done by merely adding distances.)