None of these are exact but close enough for what most folks need...
It's particularly easy to mentally multiply by a number of the form x.x
This makes two common metric to inferial conversions easy.
There are 2.2 pounds in a kilogram. (e.g. 2.2 * 23 kg = 46 + 4.6 = 50.6 lb)
There are 3.3 feet in a meter. (e.g., 3.3 * 1000 m = 3000 + 300 = 3300 ft)
Some other memory aids...
A km is ~0.6 mile; a mile is 1/0.6 ~= 1.6 km.
A US nickel weighs 5 grams and a post-1983 penny weighs 2.5 grams. An ounce is about 28 gm so five nickels and a penny. (Also, a penny has a diameter of exactly 3/4 inch.)